How was 'Sundae' derived from 'Sunday'?
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'?
According to the OED:
Origin uncertain. There exist a number of differing accounts both of the invention of the dish and of the coinage of its name.
The name is generally explained as an alteration of Sunday, either because the dish originally included leftover ice-cream sold cheaply on Monday, or because it was at first sold only on Sunday, having, according to some accounts, been devised to circumvent Sunday legislation. The alteration of the spelling is sometimes said to be out of deference to religious people's feelings about the word Sunday.
I'm not sure it's possible to get a definitive etymology.
I may just have the answer you've been looking for...
At an old drive-in/curbside style diner in Norfolk, Virginia, called "Doumar's." There's documentation about church goers in an uproar over the perils of sin and the "Ice Cream Sunday." Along with mention of the most terrifying concoction of all, soda, there's disgust over the association of the Lord's day with the apparently sinful gateway drug, ice cream. In an effort to calm the church, Doumar's claims that the spelling was altered to the spelling we now recognize, "sundae." The original sunday was not actually related to the day of the week, but was the English version of ice cream pioneer, Mr. Sontag's surname. You can still go to Doumars and read about the scandal there. Have a sundae while you're there and ponder all the sin you've avoided. ;-)
There are other words where ay/ae alternate as spellings (one that comes to mind is the personal name May/Mae), so that might have suggested the respelling of sundae.