Midnight to midnight, local time. If you want to complete this event (and again: you don't need to for your registries, this is just about getting a few DADA and red Spell books and keeping those clamouring for more events happy), you'll have to grind at least eight higher level fortresses. Snag: that won't necessarily be possible for lower level players. (That's okay-ish, you don't need those books as badly as the higher level players do.) After the Adversary Event (and back to back events as a whole) chewed through my potions reserves, high level fortresses don't seem too appealing, so for me the Portkeys for the House Hourglass fragments will probably be the best thing about this CD. Note, however, that you can select a from a variety of rare fragments you might need for your registries and by using a specific Fortress Chamber, you'll be able to get it. If your circumstances are different, that might prove nicer for you.
Further snag, and this is crucial: 1) we should assume they haven't fixed the fortress / event rewards bug. This means the guaranteed drop for the foundable at a given fortress level most likely only takes place if the first person to enter the chamber is in an active Community Day time zone. So basically for the next 46ish hours, basically don't enter one of the relevant fortress chambers first, unless the CD is live where you live. Likewise know that that's going to be an issue when you join a chamber because not everyone knows enough to or is inclined to observe that netiquette. This means folks in Oz will have better chances of things working as they should late in their day as other time ones go live, folks in Europe around mid-day, and folks in the States and Canada early in their days. Maybe we'll get lucky and it will have been magically repaired. I kind of doubt it. *shrug* It is what it is.
If you do nothing else, brew Health Potions and pickup your freebies.
ETA: ATTENTION! THERE'S AN ERROR! YOU COLLECT THE WILD BOAR FRAGMENTS FROM >> FOREST CHAMBER V <<. It's correct in the game task list, but ironically not in their own infographic. Just a heads up in case you're relying on this while fortressing.