September or October. Partially because you'll get good rates, but you'll also be more likely to get your pick of seats. I also want to be sure the apartment I want to rent at my vacation destination is available too. The best ones are always gone first. I plan my vacations two years in advance for the big ones, that way I've saved up the money before hand and know what my budget is. Amazing how $25/week for 2 years gets you 2 fun weeks in Paris!