I give 2 months notice and put the note w/their name in the cubbie or hand it to each parent and I email them as well. I also put a note on the door about 3weeks prior to the date and leave it up there for all to see. I also ask in May for each families summer vacation schedule. I too have been on the end when a parents comes up and says "we didn't know you were going on vacation". One year I said "well that's funny everyone else knows!".....
Now I make it to where there is no possible way that they can't not know!