Currently new commitfests need to be created from the admin panel, and dates need to be filled in manually. I think the easiest would probably be to have a "start commitfest" button that does:

1. Change the currently "Open" commitfest to "In Progress"
2. Create a new "Open" commitfest

Then there could also be an "end commitfest" button that does:

1. Change the currently "In Progress" commitfest to "Closed"

Obviously these buttons should only be pressable by some type of administrator role.