Many jams follow a regular schedule, but the actual event is only a few days long. In contrast, #1GAM is continuous and ongoing. Submissions are always accepted, and count toward the current calendar month.
Ludum Dare has been running since 2002, with tens of thousands of entrants at every level of proficiency. As an industry-wide event, participation carries a sense of being part of something, and there's no shortage of interest or discussion online.
The single-weekend format of most jams can be daunting to the beginner, but on the other hand a short deadline is a great motivator. InsanityJam allows two full weekends for development, plus the week between them.