-What are they actually celebrating?
-Their planet made a whole revolution around its star.
-I told you it is not an intelligent civilization.
Follow
So, they are celebrating metrology and the ability to measure that time period.
Thanks, using only local means to determine when the end of the year (for whichever definition thereof) happens seems like a nice idea for a way to celebrate it.
@PawelK IMO the reason we sometimes (imo too rarely in general) celebrate discoveries is so that we keep noticing that they were important and nontrivial, even after they've become part of the expected environment in the same way sewers are.
@robryk
Memes author night be assuming, and probably is right about it, that for distant alien civilizations this one isnt as impressive a trick.