If a major holiday was based around your character, how would it be celebrated, and why is it celebrated that way? What day does it fall on? Anything else you'd like us to know?
For bonus points, tell us how the various splinter groups (aka those lovable nutjobs) celebrate it. How is it different from the mainstream celebrations?
Emil himself isn't that special, but Ratatosk is a Summon Spirit and therefore something like a god. He controlled the flow of mana from the Great Kharlan Tree, and he's the lord of all monsters. Considering another person destined to become the next Summon Spirit-- Martel Yggdrasill-- is worshiped by the Church of Martel as a goddess, it's not too far-fetched that Ratatosk, if seen in a better light, could be the focus of a holiday/religion as well.
I think it would fall on October 31st (or the Aselian equivalent) because Ratatosk holds the gate to the demon world shut, even though he's associated with darkness and demonic things himself. There would be no candy, though. Mainly, people would celebrate it by spending a quiet evening indoors as soon as the sun went down. They'd eat a simple, homecooked meal while bathing in the light of eight multicolored candles, each representing a Centurion/one of Ratatosk's servants. If the family resided in a town, the town's elders would dress in animal skins and go around from house to house, banging on the doors, and lamenting that Ratatosk's power is preventing them from stepping foot inside the human world-- a la The Village.
The crazy people? Well, they'd probably take this to the next level. They'd try to sacrifice a member of their cult as the sun set in order to please Ratatosk and ensure that he continues to protect the gate to Niflheim forever. Thing is, Ratatosk despises humanity for killing his Tree, so it'd take more than the sacrifice of a parasite to please him.