Starting with our July 2nd dance BIDA has been running half our dances as mask-optional. Some people only want to attend if they know everyone on the floor will be wearing a high-filtration mask, others only want to attend if they don't have to mask, and of course others will or won't attend regardless. Offering some of each lets people choose the dances that fit their preferences.

Plus, as someone who likes experiments, it's almost like a randomized controlled trial of mask requirements on attendance!

Before we switched to alternating masks required and optional we didn't know what effect this would have on attendance: would there be more people on mask-optional nights (more fun!) or on mask-required nights (more safe!) Here's what I see looking back over attendance:

Note that aside from there only being eleven dances, there are some unique aspects of some of the dances that you might want to adjust for:

The 7/2 dance was the first mask-optional dance, and so might have gotten a boost from people who were excited to dance without masks but wouldn't come every time.
The 7/2 and 9/3 dances were our last dances before the summer and first dance back, which could have increased attendance. But they were also both holiday weekends, which could have decreased attendance.
The 11/5 and 11/19 dances were both double dances, which generally have somewhat higher attendance. But we (mostly by accident) did two back to back, one of each.
The 12/3 dance was a family dance, which also usually means higher attendance.
The 12/17 dance was at a different venue, the Somerville Armory, which usually means lower attendance.
The 1/7 dance was during a major snowstorm, which usually means much lower attendance.

Overall, it looks like mask-optional nights are a bit more popular, but with all the caveats it's hard to say. Regardless, the difference clearly not so large that it's a reason to stop one or the other. I expect we'll continue alternating, since we still hear from many people that they only want to attend one or the other.

(All this is speaking for myself, not the BIDA Board.)