Interational Film Festival of Thessaloniki.
It's a must everyday, for as many films as you can handle each day and lasts for one week. Everybody you met the previous day will be there. Meaning every student and tourist there are. A lot of artists and actors are present also (well not the really famous ones; its Thessaloniki, not Holywood) and its great fun. And you manage to see a lot of films really cheap or even for free if you have some good connections.
Its every November usually at the second week of the month. I know that it is not worth it if you are there for only a week, but I think it is, if you spend just a day for that.