Jack-o'-melons are used as a decoration during Summerween. They're similar to a Jack O' Lantern, except they are made from watermelons instead of pumpkins. In the summer, the Gravity Falls residents light up Jack O' Melons for the holiday of Summerween. The Trickster was going to kill Dipper, Mabel, Grenda, Soos, and Candy if they did not get 500 pieces of candy before all the melons were out.
Traditionally, Jack-o'-Lanterns are made out of pumpkins. Yet for Summerween, Jack-o'-melons are made with watermelons, hence their name. Jack O' Melons are seen throughout the town of Gravity Falls, where residential homes are decorated with them in spirit of the holiday.
Jack-o'-melons resemble Jack O' Lanterns, except Jack-o'-melons are made from watermelons instead of pumpkins. They both have faces carved into them and a lit candle inside them.
- 112. "Summerween"