Are you looking for seasons? Or times of the day?

Seasonally, mid to late summer and early fall typically gives you the biggest selection of new hatchlings.

Times of day...sort of depends on what you're trying to do. Personally, I prefer arriving at a show as early as possible. You see more animals that way, and they tend to be a bit less crowded than later in the day. If you want to wheel 'n deal...then right before closing is often a good time, because vendors are eager to move as many animals off their table as they can and are likely to give better deals then they might have at the beginning of the show.

EDIT: started typing before the poll was posted. LOL