Ability to schedule unlimited recurring sessions
eg. Every Saturday, same time, same location or Every 1st Saturday, same time, same location. Calendar then displays the upcoming sessions as far forward as the admin/user wants to filter down.
Unless it was coded from scratch, I'm surprised it wasn't in calendar scheduling software's basic functionality - it's arguably in Minimum Usable Product territory (at least for SANDARC & other VECs with regular schedules like this), as IIRC I've seen it in every calendar program on every platform.