Palms, of course, not being native to England, they used willow or yew branches on Palm Sunday. The things you learn from the OED! We find this under palm:
3.a. Freq. in pl. A branch or sprig of any of several early-flowering willows (esp. the sallows, Salix caprea and S. cinerea), esp. as substituted in northern countries for the true palm in celebrations of Palm Sunday; a branch of several other kinds of tree or shrub used in a similar way, as yew, Taxus baccata, (N. Amer.) T. canadensis, and spruce, Picea abies; (occas.) any of the trees or shrubs providing such branches. Freq. also: these branches or sprigs collectively.
The night before Palm Sunday, boys would go 'palming' to collect the branches. I wonder where they come from now? A church supplies shop, probably. I also learned from the same source that just as there's a Whitsun, so there was once a 'Palmsun', shortened from Palm Sunday:
1. Palmsun eve the day or the evening before Palm Sunday.
2. In other attrib. uses: designating things relating to or connected with Palm Sunday; spec. designating an event held on or around Palm Sunday.
As far as Google can inform me, there doesn't seem to be a Palmsun fair in Caistor any more. The OED calls this word 'regional (chiefly north.)' and not obsolete, but it surely must be now - although maybe in the north...
It was the 'willow palms' that really interested me, though. It makes quite a difference to think of crowds bearing branches of greenery, freshly gathered from the fields the night before, rather than the dried dead things we carry. It's much less authentic, of course, if you want to strictly imitate a crowd in first-century Jerusalem, but that wouldn't have mattered so much even a century ago in this country, and even less so in medieval England. Medieval Christians were very good at interpreting foreign elements of the faith in the light of their native culture (as well as vice versa). That's how medieval history works: just as Chaucer's Troy looks like fourteenth-century London, and Noah in the mystery plays talks like any medieval tradesman, a Jewish crowd might wave willow branches from the banks of English rivers. It's not lack of imagination, though in the case of the palms it probably arises from nothing more than necessity. If anything, it's ingenuity: the same process leads to the best literary effects of the period, because it involves taking complete possession of whatever story is being told, engaging thoroughly with it in every detail, and imagining it as no less fresh and living than something that happened yesterday.
Thinking about this reminded me that it's now time to turn to Piers Plowman again, as I like to do in the week before Easter. The later sections of the poem retell the story of the Passion as if it's being witnessed by Will, the protagonist, and it's full of glorious poetry which weaves the language and actions of the liturgy into a startlingly vivid narrative, all while reflecting on time and history and justice and mercy and love, and everything in between. I'll dig it out and post about it. There's so much good medieval literature about the Passion - from the Dream of the Rood to Middle English lyrics to the mystery plays, it's an endless source of inspiration for medieval poets. You couldn't read it all in a week - perhaps not in a lifetime!