Every day of the calendar has its corresponding saint. Most of these saints are minor and not well-known, but St. Ferragun's Day is a well-known favorite.
St. Ferragun is the patron saint of wrestlers. Originally, his day was a time to challenge one's neighbor to a friendly wrestling match, but now the holiday has shifted, expanded.
On St. Ferragun's Day, anyone can challenge anyone else to a fight, as long as that person is outside of their own home. People who want to fight, but don't want to leave home, will hang gongs in front of their house, so that challengers can summon them. In rural areas, fights are scheduled weeks in advance, to accommodate travel time.
The streets are filled with duels, brawls, and wrestling matches. The windows are full of spectators.
Refusing a match brings a curse. Challenging a underpowered opponent brings a curse. So does unnecessary roughness. Ferragun is a saint, after all.
The challenged person chooses the terms of the fight, usually. (You cannot challenge wizards to wrestling matches.)
Traditionally, the loser gives a belt to the victor. Beware of swaggering young men wearing a dozen belts. Every village has a badass.
The day after St. Ferragun's Day is St. Vivione's Day, the patron saint of healing.
How to Use This in Your Game
Want to wrestle the blacksmith who cheated you last session? Want to see how badass the king's champion is, in a situation where he isn't actively trying to kill or arrest you? Hell, the king's challenger might even invite one of the PCs to a friendly match in the rose garden, followed by high tea.
It's also a good day to have your players fight a procession of weird challengers (maybe one per PC) or an entire rival adventuring group. And if your party isn't interested, they can just stay in their rooms.