In the early years of the show,

'Simpsons' used to be on weeknights- Thursday nights for a few years, up until season 6.

Why was it changed from Thursday to Sunday? I heard this used to get double-digit ratings on a weeknight for a cartoon.

Actually, The Simpsons premiered on Sunday night, Dec. 17, 1989. In
August 1990, it was switched to Thursday nights. In September 1990, it
found itself opposite NBC's hugely popular Cosby Show. It managed to
remained in Fox's Thursday time slot until August 1994, when it was moved
back to Sunday nights. (By that time, The Cosby Show had run its
course, having ended on Sept. 17, 1992. So, I'm not sure why the switch
back to Sunday.)