Sundae

Wisconsin ice-cream-parlor owner George Gitty probaly first called this concoction, which he did not invent, a Sunday back in the early 1890s becasue he regarded it as a special dish only to be sold on Sundays. No one knows exactally why or when Sunday was changed to sundae.

Source:QPB Encylopedia of Word and Phrase Origins, Robert Hendrickson

Source: Dictionary of Word Origins, John Ayto