She dies in http://wiki.answers.com/wiki/Alone_Again,_Natura-Diddily which is season 11,episode 14!
Mona Leaves-a a very sad episode.
Season 11 episode 14. It's on disc 3 and it's called "Alone Again, Natura-Diddily".
Bart of Darkness. It's in Season 6.
Oh there is lots of Simpson summer episodes. I can name a few. Summer of FT42 aired in May 1996 since every season goes from September or October or November to about May