Any of the winter months.

1. You avoid the heat while you are at your largest.
2. You don't have to worry about bubs overheating overnight while he/she is a newborn.
3. You don't have to worry about sunburn with a newborn.

I honestly hadn't really thought about it before getting pregnant last time but I think it worked pretty well giving birth in late June. I suffered badly from heat early in the second tri so maybe an August of September bub would be good too.