In the episode where Bart falls for Jessica Lovejoy (the seemingly perfect Minister's daughter), you see him crossing off all the days on the calendar he feels he needs to survive to get over her.
He then says, "Alright, Day 1" and circles the first of the crossed off days. If you look carefully, you will see that the day he circled is actually a Monday. But then Marge comes in and tells him its time for Church.
Since when do they go to Church on Monday?