Bel(l)taine, aka May Day, aka the beginning of summer. Popularly held by many to be ‘Celtic’ and ‘Pagan’ and a whole lot of other stuff that it wasn’t and isn’t. Its earliest historical attestation comes from Early Medieval Ireland and up to quite recently, long held folklore traditions and customs continued in several parts of the country (as I write, the smell of smoke is drifting in the window from a nearby May Day bonfire).
The most common components of such traditions and associated folklore (and the ones which appear in the earliest references) involve fire, animal welfare/protection (especially cattle) in the hope of good yields to come – all hinting at the seasonal attributes and patterns involved in medieval economies involving transhumance. There are of course many other traditions, but these are later manifestations in subsequent centuries. For the moment, I will stick with the basic version: Vanilla Bealtaine 1.0.