Halloween in July? Welcome to ‘Summerween.’

On a hot summer day, Bonnie Barton hosted a festive backyard gathering that looked different from the typical summertime pool party.

Jack-o’-lanterns were fashioned out of melons, instead of pumpkins. A novelty skeleton chilled out in an inflatable pool. Guests played ring toss, flinging plastic orange hoops onto pointy black witches’ hats.

Ms. Barton, 35, a social media influencer in Austin, Texas, was celebrating a novel holiday: Summerween.

A mash-up of a summer celebration and Halloween, the event seems to have been assembled, Frankenstein-style, out of social media’s insatiable need for content, retailers’ desire to drum up excitement in the dead zone of July and August and an enthusiasm among certain people for ghouls and goblins so strong that it cannot be satisfied by one holiday a year.

When Did It Start?

The term “Summerween” originated on “Gravity Falls,” an animated mystery TV series that had a two-season run on the Disney Channel and is now available on Disney+.

In Season 1, episode 12, which premiered Oct. 5, 2012, the residents of the fictional town of Gravity Falls, Ore., love Halloween so much that they decide to celebrate twice a year. For the summertime version, they carve jack-o-melons instead of pumpkins and tell tales of the Summerween Trickster, a creature that eats children who don’t show Summerween spirit.