puupia: if you set vacation days, when your opponents play the game will immediatelly assume those days and include them in the time left (assuming it's not a Fischer clock game or without vacations).
Also your opponents will be able to see on your profile that you're on vacations and for how long instead of expecting you to move every day.
On certain occasions you can even "waste" less vacation days if you set them than if you let autovacations kick in (not that it matters much - we're at the end of the year and you still have 35 days!)

pauloaguia: Ok. Letting your opponents know about not playing seems like a good idea. I'll put some vacation in my calendar.

I thought my vacation days would not be reset to maximum when the year changes. There no reason to save the vacation then if they do reset..

puupia: That is correct. Every January 1, your vacation days are reset to the max. So you could have fun and set the rest of December as vacation days since they will be reset to 35. (and of course you can still be here and play your games)

And as noted, vacation days do not work with Fischer clock game, or fixed weekend only games (games with red dot next to time).