When Bart got a girlfriend in Season 6, it went terribly for him. So, in hopes of breaking the hold she had over him, he decided to just avoid her entirely.
Naturally, the joke is that he hasn't really thought this whole 'avoiding' thing through, and though he'd like to keep away from her for three months, it doesn't occur to him that they both attend the same school and church. In fact, a moment after he's revealed the joke of the scene by circling the first day he'll avoid her, Marge walks in and announces that it's time to go to church!
Funny scene and all, but I've got a question - why are they going to church on Monday?
Is it a prank on Bart (and all the children of the town) because it's also April first? Inquiring minds want to know, Simpsons!