If a game jam is Ongoing (meaning, it has already started) and lasts for more than 30 Days, then it won't be shown on the jam calendar anymore, until less than 30 Days of its duration remain, at which point it will reappear on the calendar.
After a game jam ends, it will disappear from the jam calendar too, unless it's less than 30 days since the jam started. In that case, it will remain on jam calendar until 30 Days have passed since the game jam's start.