(based on the episode "Irrational Treasure" [1x8])

You and your siblings are spending your summer holidays in Gravity Falls with your Grunkle Stanley. When you're in town, you can see that there's a holiday named "Pioneer day"

Stanley: oh god, not the Pioneer day..

Mabel: Pioneer day? What's that?

Stanley: that's the day, when Gravity Falls celebrates the founding of the town by Nathaniel Northwest... Many festivities are included with Pioneer Day to make it seem like 1863... And if you came back home and started to say "Hornswaggle", I will disinherit you!