Mostly for my own reference but...
Under this cut is a timeline of events from the Durarara novels (volumes 1-6), cut down to only events relevant to Kida. There are also links to scene translations where available. The dates aren't exactly matching with the anime timeline, since the anime's spreading things out over a more reasonable amount of time, but the general flow of events is correct, as far as I can tell.
All events that have not happened/have not been STRONGLY implied in the anime as of the end of the anime series have been whited out, so anyone following only the anime should be okay to read this so long as they don't try to highlight the post.
Legend:
+ = Events that have happened
? = Events that have not happened (yet)
Strike-outs = Events that did not and will not happen.
Ca. year -16
+ Births of Ryuugamine Mikado, Kida Masaomi, Sonohara Anri, Harima Mika, and Yagiri Seiji.
Year -4
+ Kida Masaomi transfers to an elementary school in Tokyo, but stays in online contact with Ryuugamine Mikado.
Ca. year -2
+ The gang taking shape around Masaomi begins to be referred to as Yellow Scarves.
+ Mikajima Saki approaches Masaomi under Izaya's instruction and becomes Masaomi's girlfriend. Izaya himself approaches Masaomi at the same time.
Year -1
+ Conflict erupts between Yellow Scarves and Blue Square, with Izaya selling information to both sides.
+ Saki is abducted and injured by Blue Square under Izaya's instructions. A panicked Masaomi calls Izaya for help but gets no reply. Blue Square members Kadota, Yumasaki and Karisawa betray their gang and save Saki, who is immediately hospitalized.
+ Ashamed of himself, Masaomi breaks up with Saki and leaves Yellow Scarves.
+ Masaomi invites Mikado to go to Raira Academy with him.
Afterwards (period unknown)
+ Color Gangs continue to cause unrest for a while before gradually going out of existence.
+ Blue Square dabbles in drug dealing, which draws hostile attention from Shiki of Awakusu-kai. Shortly afterwards Blue Square get into trouble with Shizuo, leading to its complete disintegration. Remaining members like Horada and Higa are absorbed by Yellow Scarves.
Year 0 (This is where the anime begins)
Day before school opens
+ Mikado arrives in Ikebukuro. Masaomi and Mikado sees Celty as Masaomi is showing Mikado around Ikebukuro in the evening.
School-opening day (possibly April)
+ Mikado, Masaomi, Anri and Seiji attend school-opening ceremony, with Mika absent.
+ Masaomi shows Mikado around Ikebukuro. The two meet Yumasaki, Karisawa, Simon and Izaya.
2nd day after school opening
+ Physical examinations and class meetings at Raira Academy. Mikado and Anri become the male and female class representative respectively.
- Masaomi asks Seiji to join him and Mikado to pick up girls. Seiji refuses saying that he cannot betray his love.
+ Masaomi and Mikado find Anri cornered by bullies. Izaya, who came looking for Mikado, appears from behind and scares the bullies away by stomping on one of the girls' cell phone. Shizuo hits Izaya with a flying trashcan. The two fight until Simon stops them.
Period unknown (shortly after Namie's flight)
+ Several pistols are stolen from Awakusu-kai. Horada steals one pistol from the thief when the thief is intoxicated. All except for that one pistol are retrieved and returned by Celty.
Period unknown (middle of year)
+ Yellow Scarves begin to absorb members in Tokyo.
Year 1 (Start of episode 13, post six-month timeskip)
Shortly before finals at Raira Academy
+ Masaomi catches Nasujima trying to harass Anri, and thereby extracts from Nasujima part of the problems that are going to be on the final exam.
+ Yellow Scarves move into the open on Ikebukuro streets.
Early March
Date unknown
+ Celty buys information from Izaya on the demon blade Saika. As Celty takes Shizuo to find and kill Saika, Shizuo beats up several Yellow Scarves who picked on them.
+ Fifty-four Yellow Scarves become victims of Saika at the same time. (This is later referred to as Ripper Night and triggered the conflict between Dollars and Yellow Scarves)
Afterwards
+ Mikado and Masaomi visit Anri in hospital. Anri confesses to Mikado that she is vaguely attracted to Celty.
+ Yellow Scarves force Masaomi to become their leader again and swear revenge against the culprits of Ripper Night.
One week before commencement (several weeks after Ripper Night)
+ Masaomi visits Saki in the hospital. Saki believes it to be a sign that Masaomi is coming back to her because Izaya told her so.
+ Masaomi deserts Mikado and Anri during an outing, and meets with Yellow Scarves members to discuss the matter of Horada being injured by Shingen. Yellow Scarves suspect Dollars to behind Ripper Night and plan to seek revenge. Anri follows Masaomi and finds out that Masaomi is the leader of Yellow Scarves.
+ Celty saves Anri when she is about to be seen by the Yellow Scarves. She makes a shadow helmet to conceal Anri's face. Anri uses Saika to cut the objects thrown to them by Yellow Scarves and escapes successfully with Celty.
Day before commencement
+ Masaomi visits Saki again in the hospital.
+ Masaomi meets with Kadota, Yumasaki and Karisawa in Russian Sushi hoping to clear the misunderstandings between Dollars and Yellow Scarves. Yumasaki is majorly displeased when Masaomi presses for information about where former members of Blue Square went. Kadota directs Masaomi back to Izaya, who takes this chance to tell Masaomi that Mikado is the leader of Dollars.
+ Izaya uses Higa to leak Shizuo's information to Horada, leading to Shizuo and Tom being cornered by a gang of Yellow Scarves in an alleyway. When Shizuo is about to defeat the Yellow Scarves, Horada shoots Shizuo in the upper body and foot with his stolen pistol, and lies to Tom saying he did it on Masaomi's orders.
Commencement day
+ Saki calls Russian Sushi from her hospital phone and informs Simon and Dennis of what Izaya is up to.+ Shizuo wakes up at Shinra's apartment after his surgery and swears to kill Masaomi. Anri realizes that Celty is female.
+ Namie informs Horada of the close friendship between Masaomi and Mikado despite Mikado being the leader of Dollars. Horada declares Masaomi a traitor and leads his faction to seize power for himself. Masaomi fights the Yellow Scarves alone.
+ Yellow Scarves who are also Saika's children turns to Masaomi's side at Anri's order. Kadota and some thirty Dollars members pretending to be Yellow Scarves engage other Yellow Scarves in battle and defeat them. Shizuo slashes the top of Horada's car off with a road sign as Horada tries to drive away. Horada is later arrested by the team of traffic police led by Kuzuhara.
Afterwards
+ Masaomi drops out of Raira Academy, gets back together with Saki and the two start working for Izaya.
April
Date unknown
+ Masaomi joins the online chatroom at Izaya's invitation to keep in touch with Mikado.
END OF ANIME
Late April
? On Izaya's orders, Masaomi and Saki leave for a Northeastern town for a week and collect information on Yadogiri Jinnai.
May 2nd (Night)
? Izaya impersonates Masaomi in the chatroom and warn Mikado to try not have anything to do with Dollars for a while.
May 3rd (Day)
? Masaomi and Saki return from the Northeastern town.
May 4th (Morning)
? Masaomi finds out that the chatroom logs have been deleted (by Izaya).
(Around noon)
? Masaomi realizes that Izaya has been impersonating him in the chatroom and calls Izaya to ask what Izaya has said to Mikado. Izaya avoids the question and tauntingly suggests that Masaomi get in contact with Mikado himself.
(Around evening)
? Mikado tells Chikage that he is the founder of Dollars and is willing to take full responsibility for the things that happened. Chikage doesn't take it seriously and suggests that Mikado stay away from anything to do with Dollars. Masaomi, who has returned to Ikebukuro and witnesses this scene, fails to bring himself to face Mikado.
All credit for timeline/translations/etc goes to anni_fiesta who is amazing. The original timeline can be found here (WARNING: CONTAINS SPOILERS FOR THE NOVELS).
All events that have not happened/have not been STRONGLY implied in the anime as of the end of the anime series have been whited out, so anyone following only the anime should be okay to read this so long as they don't try to highlight the post.
Legend:
+ = Events that have happened
? = Events that have not happened (yet)
Strike-outs = Events that did not and will not happen.
Ca. year -16
+ Births of Ryuugamine Mikado, Kida Masaomi, Sonohara Anri, Harima Mika, and Yagiri Seiji.
Year -4
+ Kida Masaomi transfers to an elementary school in Tokyo, but stays in online contact with Ryuugamine Mikado.
Ca. year -2
+ The gang taking shape around Masaomi begins to be referred to as Yellow Scarves.
+ Mikajima Saki approaches Masaomi under Izaya's instruction and becomes Masaomi's girlfriend. Izaya himself approaches Masaomi at the same time.
Year -1
+ Conflict erupts between Yellow Scarves and Blue Square, with Izaya selling information to both sides.
+ Saki is abducted and injured by Blue Square under Izaya's instructions. A panicked Masaomi calls Izaya for help but gets no reply. Blue Square members Kadota, Yumasaki and Karisawa betray their gang and save Saki, who is immediately hospitalized.
+ Ashamed of himself, Masaomi breaks up with Saki and leaves Yellow Scarves.
+ Masaomi invites Mikado to go to Raira Academy with him.
Afterwards (period unknown)
+ Color Gangs continue to cause unrest for a while before gradually going out of existence.
+ Blue Square dabbles in drug dealing, which draws hostile attention from Shiki of Awakusu-kai. Shortly afterwards Blue Square get into trouble with Shizuo, leading to its complete disintegration. Remaining members like Horada and Higa are absorbed by Yellow Scarves.
Year 0 (This is where the anime begins)
Day before school opens
+ Mikado arrives in Ikebukuro. Masaomi and Mikado sees Celty as Masaomi is showing Mikado around Ikebukuro in the evening.
School-opening day (possibly April)
+ Mikado, Masaomi, Anri and Seiji attend school-opening ceremony, with Mika absent.
+ Masaomi shows Mikado around Ikebukuro. The two meet Yumasaki, Karisawa, Simon and Izaya.
2nd day after school opening
+ Physical examinations and class meetings at Raira Academy. Mikado and Anri become the male and female class representative respectively.
- Masaomi asks Seiji to join him and Mikado to pick up girls. Seiji refuses saying that he cannot betray his love.
+ Masaomi and Mikado find Anri cornered by bullies. Izaya, who came looking for Mikado, appears from behind and scares the bullies away by stomping on one of the girls' cell phone. Shizuo hits Izaya with a flying trashcan. The two fight until Simon stops them.
Period unknown (shortly after Namie's flight)
+ Several pistols are stolen from Awakusu-kai. Horada steals one pistol from the thief when the thief is intoxicated. All except for that one pistol are retrieved and returned by Celty.
Period unknown (middle of year)
+ Yellow Scarves begin to absorb members in Tokyo.
Year 1 (Start of episode 13, post six-month timeskip)
Shortly before finals at Raira Academy
+ Masaomi catches Nasujima trying to harass Anri, and thereby extracts from Nasujima part of the problems that are going to be on the final exam.
+ Yellow Scarves move into the open on Ikebukuro streets.
Early March
Date unknown
+ Celty buys information from Izaya on the demon blade Saika. As Celty takes Shizuo to find and kill Saika, Shizuo beats up several Yellow Scarves who picked on them.
+ Fifty-four Yellow Scarves become victims of Saika at the same time. (This is later referred to as Ripper Night and triggered the conflict between Dollars and Yellow Scarves)
Afterwards
+ Mikado and Masaomi visit Anri in hospital. Anri confesses to Mikado that she is vaguely attracted to Celty.
+ Yellow Scarves force Masaomi to become their leader again and swear revenge against the culprits of Ripper Night.
One week before commencement (several weeks after Ripper Night)
+ Masaomi visits Saki in the hospital. Saki believes it to be a sign that Masaomi is coming back to her because Izaya told her so.
+ Masaomi deserts Mikado and Anri during an outing, and meets with Yellow Scarves members to discuss the matter of Horada being injured by Shingen. Yellow Scarves suspect Dollars to behind Ripper Night and plan to seek revenge. Anri follows Masaomi and finds out that Masaomi is the leader of Yellow Scarves.
+ Celty saves Anri when she is about to be seen by the Yellow Scarves. She makes a shadow helmet to conceal Anri's face. Anri uses Saika to cut the objects thrown to them by Yellow Scarves and escapes successfully with Celty.
Day before commencement
+ Masaomi visits Saki again in the hospital.
+ Masaomi meets with Kadota, Yumasaki and Karisawa in Russian Sushi hoping to clear the misunderstandings between Dollars and Yellow Scarves. Yumasaki is majorly displeased when Masaomi presses for information about where former members of Blue Square went. Kadota directs Masaomi back to Izaya, who takes this chance to tell Masaomi that Mikado is the leader of Dollars.
+ Izaya uses Higa to leak Shizuo's information to Horada, leading to Shizuo and Tom being cornered by a gang of Yellow Scarves in an alleyway. When Shizuo is about to defeat the Yellow Scarves, Horada shoots Shizuo in the upper body and foot with his stolen pistol, and lies to Tom saying he did it on Masaomi's orders.
Commencement day
+ Saki calls Russian Sushi from her hospital phone and informs Simon and Dennis of what Izaya is up to.+ Shizuo wakes up at Shinra's apartment after his surgery and swears to kill Masaomi. Anri realizes that Celty is female.
+ Namie informs Horada of the close friendship between Masaomi and Mikado despite Mikado being the leader of Dollars. Horada declares Masaomi a traitor and leads his faction to seize power for himself. Masaomi fights the Yellow Scarves alone.
+ Yellow Scarves who are also Saika's children turns to Masaomi's side at Anri's order. Kadota and some thirty Dollars members pretending to be Yellow Scarves engage other Yellow Scarves in battle and defeat them. Shizuo slashes the top of Horada's car off with a road sign as Horada tries to drive away. Horada is later arrested by the team of traffic police led by Kuzuhara.
Afterwards
+ Masaomi drops out of Raira Academy, gets back together with Saki and the two start working for Izaya.
April
Date unknown
+ Masaomi joins the online chatroom at Izaya's invitation to keep in touch with Mikado.
END OF ANIME
Late April
? On Izaya's orders, Masaomi and Saki leave for a Northeastern town for a week and collect information on Yadogiri Jinnai.
May 2nd (Night)
? Izaya impersonates Masaomi in the chatroom and warn Mikado to try not have anything to do with Dollars for a while.
May 3rd (Day)
? Masaomi and Saki return from the Northeastern town.
May 4th (Morning)
? Masaomi finds out that the chatroom logs have been deleted (by Izaya).
(Around noon)
? Masaomi realizes that Izaya has been impersonating him in the chatroom and calls Izaya to ask what Izaya has said to Mikado. Izaya avoids the question and tauntingly suggests that Masaomi get in contact with Mikado himself.
(Around evening)
? Mikado tells Chikage that he is the founder of Dollars and is willing to take full responsibility for the things that happened. Chikage doesn't take it seriously and suggests that Mikado stay away from anything to do with Dollars. Masaomi, who has returned to Ikebukuro and witnesses this scene, fails to bring himself to face Mikado.
All credit for timeline/translations/etc goes to anni_fiesta who is amazing. The original timeline can be found here (WARNING: CONTAINS SPOILERS FOR THE NOVELS).