@TweeterWill83 Icecream sundae on Sunday! (@ mcdonalds, Drapery Street, Northampton) http://4sq.com/ap1SdK
There seem to be a number of stories flying around the internet, which makes picking out the true one (if it is indeed any of them) a little bit difficult, but most of them seem to have a similar theme, and that theme starts off with ice-cream soda.
An ice-cream soda, also known in various parts of the world as a float, brown/black cow or spider*, is a beverage made with any number of scoops of ice cream (of any flavour, though classically vanilla) dolloped into a glass of coke, lemonade or any other soft drink**. Originally, it seems that most were made by scooping ice-cream into plain soda water, and then adding a flavoured syrup.
This new treat became popular in its own right and started to be requested and sold on days other than a Sunday. The name seems to have been altered to 'sundae' for two reasons: as it was no longer just sold on Sundays, the name 'sunday' didn't fit any more, and apparently a number of religious-types took offence at the name of their sabbath being used as the name for a wickedly pleasurable dessert.
There are a number of American towns that fight for the right to claim invention of the sundae, including Evanston, Illinois (invention claim date: 1890), Ithaca, New York (invention claim date: 1892) and Two Rivers, Wisconsin (invention claim date: 1899), all of which have slightly different stories with their own convincing proofs and dubious inconsistencies!
* And, in the rather specialised parts of the world that contain my immediate family, a 'grandma special'.
** I'm fairly sure that traditionally, coke is used, but a 'grandma special' usually involves lemonade, and some versions even use beer and other alcoholic drinks.