I was curious so I did some research:
This is the share of the annual box office for each calendar month. From looking at the trends Jan and October are terrible months. March, September & December have become more relevant. May has somehow become less important. The others seems to be roughly the same as they always were.
May and April are the months mostly likely to be dominated by one movie - both average 38% of all receipts coming from just one property. December is 33%, February, June & July 30%, the rest lower.
One major change is the domination of a single movie over the years. In 2012 one movie averaged 26% of monthly ticket sales. Same held true in 2013, up to 27% in 2014. In 2015 that number jumped to 34% and it’s been close to there since. It’s pretty clear box office is becoming more controlled by these big Disney properties. I’m sure that’s their plan.