Hey, Thanlis! Thank you for asking! I got a similar question and answered it here. The gist of it is that we wanted to encourage experimentation. But you're the second person to ask, so I realize that we probably made it too constraining.
So, if the jam goes well and we make another, we already decided to make it longer next time! (Not this time, because it doesn't feel fair to change the dates after a few people already joined).