Has ZOS mentioned anything about holidays yet? The tie-ins to the Elder Scrolls lore would seem to be a no-brainer.
Check out the various events and holidays on the Tamrielic calendar. I'd bet most people would be surprised how many there are and what they signify. For example, Breton's have Fishing Day on the 20th of Second Seed, Harvest's End is on the 27th of Last Seed, the Witches' Festival is on the 13th of Frostfall. Take a look at your favorite TES wiki and go down the list. A treasure trove of dates in the lore to build events and quests around in ESO.