Filling out your regional info in you avatar to the left helps everyone. It's an important piece of info that is crucial to getting your questions answered accurately.
If you're talking about N.E. USA, the Nationals show is every other year in upstate New York (biggest show on the east coast), Mid Atlantic Bonsai Societies (MABS) also has a relatively large spring festival.
There are a number of smaller regional shows--the Potomac Bonsai Association show is this weekend in Washington D.C. There are a few others, but they're mostly being held this month or last in early-mid spring. There are some small regional shows that exhibit in the fall and winter. The Winter Silhouette show in December in N.C. is probably the most attended.