W is for Walpurgisnacht: Zelazny A to Z

Walpurgis Night, an abbreviation of Saint Walpurgis Night (from the German Sankt Walpurgisnacht [saŋkt valˈpʊʁɡɪsˌnaχt]), also known as Saint Walpurga's Eve (alternatively spelled Saint Walburga's Eve), is the eve of Christian feast day of Saint Walpurga, an 8th-century abbess in Francia, and is celebrated on night of 30 April and the day of 1 May. This feast commemorates the canonization of Saint Walpurga and the movement of her relics to Eichstätt, both of which occurred on 1 May in the year 870.

You'd think Walpurgisnacht would be an Amber story, as it's an unusual word that Merlin employs several times in Trumps of Doom, but it's a story about a computerized tombstone and a curmudgeonly uncle.

From the same article:

30 April is pálení čarodějnic ("burning of the witches") or čarodějnice ("the witches") in the Czech Republic. Huge bonfires—up to 8 metres (26 ft) tall—are built and burnt in the evening, preferably on top of hills. Young people gather around. Sudden black and dense smoke formations are cheered as "a witch flying away". An effigy of a witch is held up and thrown into a bonfire to burn.As evening advances to midnight and fire is on the wane, it is time to go search for a cherry tree in blossom. Young women should be kissed past midnight (and during the following day) under a cherry tree. They "will not dry up" for an entire year. The First of May is celebrated then as "the day of those in love".

My last name is Czech. Now I finally understand why I feel the urge to set Merlin on fire every April 30th.