Volume 1 of Stan Sakai's classic Usagi Yojimbo, the tale of a wandering rabbit samurai.
Ronald Hutton's Stations of the Sun, a handy one-volume woo-free history of British ritual and folk custom.
So let's see what we've got! Opening Usagi Yojimbo to a random story, we get a tale in which Miyamoto Usagi finds himself resting at an inn full of drunken gamblers celebrating.
And in Hutton we've got a discussion of violence on St David's Day in the 16th and 17th centuries (English prejudice against the Welsh was a little more severe back then because there weren't enough Irish people to go around).
To the Welsh, of course, it was a matter for pride, symbolized by the wearing of a leek as a national emblem. It had been associated with St David since the Middle Ages ... the reasons for this are lost. What was always clear was that the sporting of it was always much more common among the Welsh when abroad than at home. ...
The same observer also recorded the response of the cockneys, 'not only by calling after them Taffey, Taffey or David, David, but also by hanging out all kinds of dolls and scarecrows with leeks on their head'. Five years later, in the City, Samuel Pepys encountered '(it being St David's Day) the picture of a man dressed like a Welshman, hanging by the neck upon one of the poles that stand out at the top of one of the merchant's houses, in full proportion and very handsomely done;.
Not surprisingly, all this gave rise to a great deal of fighting, so that in 1661 alone on Welsh gentleman stabbed a local commoner near Westminster, while the retinue of another got into a pitched battle with a London crowd.OK, so this writes itself, partly because Usagi Yojimbo always has very D&D-able stories -- it's about a dude who wanders from town to town looking for trouble, after all. So our heroes arrive in a big city -- it works best when it's a big city -- which has a minority or faction who are celebrating their special feast day (bonus points if they're cranky and hungry because their feast day is in Lent).
1 - faction-identifying doodad coincidentally similar to distinctive item worn by PC (amulet, odd-coloured tie, animal familiar, etc.).
2 - drunk mob of Faction A insist that PCs join them in toast to downfall of Faction B, preferably within hearing of outnumbered and resentful B group.
3 - drunk mob of Faction B identify PC most likely to appear like member of their faction and berate him/her for lack of national/ethnic/religious/whatever spirit (e.g. "no real dwarf doesn't plait his beard on Saint Axebeard's Day!").
4 - innocuous charity-collecting process associated with celebration difficult to distinguish from being mugged by band of lunatics.
5 - restful stay at inn disrupted by boozy revelry; boozers interpret complaints as ethno-religious disparagement.
6 - neighbourhood/village bigwig offers prize for best-constructed offensively threatening caricature.
7 - traditional songs and dances performed on ethnic feast day encode legendary/magical knowledge now lost to modern culture but recognisable to e.g. Bard / PC with high Cthulhu Mythos.
8 - seemingly innocuous bardic song understood as containing coded references to ethnic tensions.
9 - embattled members of either faction offer to hire tough-looking strangers to guard / assist with their festival processions. Big fight with antagonists in antic disguises.
10 - wandering swordsman with sore feet has had 9 encounters with drunk faction mobs and will stab a fool at encounter 10.
11 - traditional Faction B festival costumes provide convenient opportunity to infiltrate thing worthy of infiltration but invite assaults from drunken mobs of Faction A.
12 - local muckamuck wants prominent member of Faction B dead/arrested but too cowardly to risk offending on feast day; hires ignorant out-of-town bounty hunters instead.