There are three larger primary exchanges every year: Secret Santa in December, Arbitrary Day in June, and Halloween in October. Look for information to start popping up early December, May, and September respectively.
There are also tons of other fun, interest specific exchanges throughout the year. If you mark your profile and make sure the box beside “Receive upcoming exchange notifications?” is checked, you will get an email when they are announced.
You can also view a Google calendar of the upcoming exchange announcements, matching days, and shipping deadlines by clicking here.
Please note that matching happens at different times of the sign up close date for each exchange. This generally happens between 8 a.m. and noon Pacific, so be sure to sign up and link your account and validate your email address before the close of sign ups.