leap year determination
This is a very easy mistake to make when bold text, dim text collide with a reasonable assumption about something as familiar as a calendar. We train ourselves to expect Sunday first or Monday first. 

In this case the solution is to get a screen shot and review the actual data, not what we think we are seeing. I say we since I’ve been in the same situation as OP. 

In the images, it’s clear that February 29 was drawn and that there’s no leap year error on Catalina here in the Gregorian calendar setting.