@dragon its the 13th for 28 day months, of which there’s only one left (but also march has always been a 31 day month with its ides on the 15th, so we fortunately don’t have to resolve the ‘but if it used to be a 28 day month…’ question)

https://penelope.uchicago.edu/~grout/encyclopaedia_romana/calendar/juliancalendar.html#anchor1263546