The first performance of a new group is kind of like a Band-aid. Best to tear it off fast.
My personal preference for first performance includes two big criteria: 1, slightly before they feel ready; 2, in front of an audience without astronomical expectations.
One great solution is to book a performance of the National Anthem at a school sporting event. A couple weeks before your first concert, a performance before a soccer game is a great self-check. What do we sound like when we perform? What issues came up when we were in front of a crowd?
Your students then have a couple weeks left to develop their concert works, and their first concert won’t be their first performance.
They’ll walk onstage with extra confidence, having already torn off the Band-aid.