They usually happen three times a year, a big one sometime in April, and others in August and December. And there's a mini-LD every month. There's a "gamecomp

They usually happen three times a year, a big one sometime in April, and others in August and December. And there's a mini-LD every month. There's a "gamecompo" mailing list on the front page of the ludumdare hub site that you can sign up for, it just sends reminders a week or so before each compo.