In pious New England which was during the 1880s, the church were furious with local shops selling ice creams in Sundays. Which was suppose to be the lord’s day, the enjoyments overshadowed the lord’s pain that we were to suppose. The church banned the treat for sundays. So sellers, decided to keep everything same from the ice cream but just change the name.. to sunday. The name was a marketing scheme which referred to Sunday the lord’s day but not the lord’s name. So since then the treat caught on and became popular.
Though at the time the two treats had no difference other than the name, now they do. Sundae’s are like a cocktail, just the cream and toppings, no cone unlike the Ice Cream, which is often in a cone.