I checked, and I'd put it up last year, July 30th: Mason bee, pollen bee or solitary bee nest box.
#13 hatched out May 12 (laid Aug. 24)
#20 hatched May 13 (laid Aug. 24)
#6, 7, 8 were laid in on Aug. 20, and appear to be getting darker.
#9 was done the same day, but it is a mason wasp!
#7 gone May 19.
#8, # 18 May 21
The last two have not hatched out. I suspect the larger ones are mason wasps.