Looks like I failed to post a submission report for May. Sorry about that. Here's a combined report for May and June.
In May, the Strange Horizons fiction department received only 328 valid submissions, tied for 23rd-highest-volume month ever. The reason volume was so (relatively) low is that we closed to subs for nine days at the end of the month. We averaged nearly 15 stories a day on the days we were open.
In June, we received 571 valid submissions, an average of 19 stories a day; the second-highest-volume month we've ever had. Volume started out very high when we reopened: 50 stories on the first day back, the second-highest-volume day we've ever had; then 37 stories the second day; then 30 the third day. It eased off a bit after that, but the first week of June was nonetheless the highest-volume week we've ever had (176 stories), and volume continued unusually high throughout the month—about 16 and a half stories a day for the last two weeks of the month.
Unfortunately, this means that the one-week closure didn't work as well as I'd hoped. It did give us a much-needed break from submissions, which was great. But I had been hoping that since we were closed for only nine days, there wouldn't be a huge flood afterward.
Across all the days we were open in May and June combined, we averaged over 17 stories a day, which is still quite high for us.
Dividing the total number stories in May and June combined by the total number of days, including days when we were closed, yields 14.7 stories a day, about as many as we received in April. So one way of looking at this is that we got the same number of stories we would have gotten if we hadn't closed.
Which I suspect means that almost everyone who showed up to submit during the closure looked at the dates and said "Oh, they'll be reopening in n days (n < 10), so I'll just wait and submit then." Whereas when we're closed for longer, presumably some percentage of people say "I'm not gonna wait two weeks/two months to submit this story, I'm gonna go submit it somewhere else."
Anyway, I was hoping that (in addition to the main effect of giving us a little breathing room) this might be a way to reduce the total number of submissions, but clearly not.