I looked up when Justice league #40 was going to come out, because It had been over a month since #39

I found that it was not out until April the 29th, and that #41 would be three months after that on the 17th of July..

what is with this? I wonder if it is something to go with Convergence, and Darkseid War?

41 is coming out on the 17th of June not July

It is still a monthly.

40 would be an extra sized issue by many artists so it needs extra time.

From 41 and on, the series will return to schedule.