What kind of day is "Nacho Sunday"?
The correct phrase here is "themed nights." Themed nights are quite common in Western countries and usually mean when a party or event has a special theme or food. For example, 80s nights, Taco Tuesdays, and so on. In this case, the speaker is saying that they need to eat nachos on Sunday. Ex. It's Taco Tuesday tomorrow, so make sure you come hungry. Ex. Why doesn't Fried Chicken Friday exist?