Advance notice would seem to be the way to go! Depending on whether or not the bannermakers are also writers, it might even be doable to provide the month's prompts and dates in advance, and let bannermakers decide what works best for their style/comfort level as well as schedule, but I'll have to think about how to do that.

Sounds like a monthly chat post might be fun! I'll see what I can do--maybe not this week, since work's kind of hitting like a ton of bricks currently, but this weekend maybe.