...hmmm, i wonder how comes you cannot have this option, to announce vacation days (or i didn't discover yet)? it is unpolite to go just like that and say nothing, and let the others guessing that your vacation has started!
message to all your opponents might be an option but, i think i should have this possibility, to make this annoucement once and for all!
You can just change your name while on vacation.
When your opponents faces 'Gabriel Ion (on vacation)' I guess they will realize you will most likely not move for awhile:)
I did this in june, and had the impression that some players that mostly move slow, suddenly moved fast to make sure I lost vacation days during that time... I would also to tell I don't play at weekends but I'm afraid people might start moving on friday evening. I feel it abusing the system.
(oops I told about the weekends anyway)