Our account bills every four weeks, not by the month. In 2021 we received 13 bills, which necessarily means that two must fall in the same calendar month. ... Our billing dates were: 23, 20, 20, 17, 15, 12, 10, 7, 4, 2, 30, 27, 25. ...
Same for me, exactly 5 days earlier EVERY month. In addition to "two must fall in the same calendar month", means also that (as
emcampbe wrote) ONE calendar month will have 2 bills.