The Calendar

[kharille]

Is there a default starting date? I glanced at the wiki and noted something about how starting dates may change depending on the last game you played? Are there any seasonal changes? Festivals?

Just curious as to whether it has any effect on the game...

[smr1957]

The starting date is always the same (the 17th of Last Seed), unless you use a mod that changes it.

And while the lore does mention a number of festivals and holidays, they are not in the vanilla game. There is, however, this mod (which also allows you to set the start date):

Holidays - https://www.nexusmods.com/skyrimspecialedition/mods/1533

So, without that mod, there is no effect on the game as far as start date or festivals/holidays.