Until about 40 years ago it was illegal in the UK to sell many things including ice cream on Sundays; however restaurants could operate and could sell desserts. These desserts sold on Sundays could not consist entirely of ice cream but could be ice cream with other things added, initially called a "Sunday special". Certain religious organisations protested about the use of the name of the Lord's Day on a commercial product, so the spelling of the name was changed in a way that did not affect the pronunciation.
