Sundae \SUHN-dey\ or \SUHN-dee\ , noun;
1. Ice cream served with syrup poured over it, and often other toppings, as whipped cream, chopped nuts, or fruit
This American English word dates to 1897 and is thought to be a play on Sunday. The spelling change is probably because of the religious nature of Sunday, though why the ice cream treat is linked to that day name is uncertain. It has been posited that sundaes were "ice cream left over from Sunday and on sale later."
There's a commercial on these days that says something like, "It's like they took all the good things and saved them for Sunday." Maybe that's the link...