There’s nothing like an all-inclusive vacation in Big Ambergris Cay to revive you, especially when you know what to expect from the elements. Here’s some info to help you choose when to book: The hottest months are usually September and August with an average temp of 83°F, while the coldest months are March and February with an average of 77°F. February and March are the driest months while September and October are the rainiest months.