Freezing traditional Greek dishes
Moussaka freezes fine, some would say it improves the flavor, and shouldn't damage the texture. 

Spanakopita I would not freeze whole, you will likely lose all the crispness and texture from the pastry. You could make the filling in advance though, and then put the pastry on the thawed filling on the day and bake it. Just be sure the filling is completely thawed, and getting close to room temperature. Also, freezing may force water out of the filling, drain it out before you bake or it could get soggy. 

Dolmades I'm not sure, I would expect they'd freeze fine, although you may get some sogginess. I'll defer this to another answer.