On Sunday, April 3,2011, Google displayed a commemorative graphic for the 119th anniversary of the first documented case of the Ice Cream Sunday.
(Image comes from: http://www.google.com/logos/2011/icecreamsundae11-hp.jpg)
The Wiktionary entry for 'Sundae' says only that it is a modification of 'Sunday' and contains a link to the Wikipedia page. The Wikipedia entry for 'Sundae' contains a bit of history of the ice cream sundae, including competing claims for the origin of the ice cream sundae, but does not detail how 'Sunday' was changed to 'Sundae'. One origin story claims that the sundae was created to comply with Blue Laws in that locality, and the name 'Sunday soda' was changed to 'sundae' after demand caused the owners of a soda shop to serve the treat on days other than Sunday. The first documented case of the sundae used the 'Sunday' spelling.
How did the spelling officially change from 'Sunday' to 'Sundae'?