Edit planner events, and get an email reminder beforehand

Planner events can now be edited after the fact. If plans change, the organizer (or a playgroup admin) can update the location, time, format, cube, max participants, or notes on an existing event instead of cancelling it and starting over. If you're signed up for a session, you'll now also get an email reminder before it starts, alongside everyone else confirmed for that slot. Note: this depends on someone visiting the Planner page around the right time to trigger it, so for a very quiet playgroup a reminder might arrive a bit late.