The Deal of the Day is a special offer where one premium item is taken and sold for half-off. This item changes every 8 hours. The Deal of the Day is found under the recomended section in the shop, and is usally the first thing to pop up.
While not only giving you an item at 50% off, this deal can be used to make the Gumball Machine more profitable. The Gumball Machine gives you one candy every 8 hours, or 3 candies a day. It normally costs 150 candies; however during the Deal of the Day, it costs only 75 [candies]. This investment would normally take 50 days (at the minimum) to break even on. Everything else past there is just profit. However, when you purchase the Gumball Machine as the Deal of the Day, it only takes 25 days (at the minimum) to pay off. That is 25 extra days profit!