I was originally thinking that the drop on the 31st could be explained by the last day of the month not counting for quotas, so they could be tallied or something. Then I realized that the last day of the month varies, and that it if that was the cause of the massive drop on the 31st, it would have made a bigger impact on the 30th.

I might do an update with some of the suggestions here, including a graph of tickets / days until end of month. [1]

[1] https://news.ycombinator.com/item?id=4666271