Book in advance like a couple of months. Waiting until near the time you want to go would cost extra. You could save couple hundred when you book in advance. Don't do the vacation near any major holiday. This is when the hotels and airports will charge you the most because of the supply and demand and because of economics. I can't say which will be the cheapest, or when. It just a matter of whom and how far in advance it is. Also before you do book the hotel, do some internet searching on the area you will be staying at. You don't want to book a hotel in a high crime area where you or your family could get robbed from. So do your research. I know the research can sound mind boggling, but it is worth it to protect your family. Hopes your vacation turns out for the best.