Walpurgisnacht refers to Walpurgis Night, a traditional spring festival celebrated on the night of April 30th into May 1st across Central and Northern Europe. It's a night steeped in folklore, often tied to witches, bonfires, and old pagan rituals, which gives the word a mysterious, almost magical weight.
In this song, the repeated cry "In der Walpurgisnacht!" (On Walpurgis Night!) makes the night itself feel like the main character of the song. Repeating the word builds a wild, ritualistic atmosphere that pulls the listener straight into that ancient celebration.
Walpurgisnacht is the night of 30 April, when German folklore says the witches gather on the Brocken to dance the winter out. Faun write it as an invitation rather than a warning. The song opens on things climbing into the evening sky — "Wildes Volk und Liliths Art" (wild folk and Lilith's kind) — and then stops describing the night and starts calling you into it: "Lasst uns zu den Feuern streifen" (let us wander to the fires), "Lasst uns mit den Funken übers Feuer springen" (let us leap the fire with the sparks). The imperatives in the chorus are the whole song. "Hört die Geigen" (hear the fiddles) and "Folgt dem Reigen" (follow the round dance) are addressed to someone standing at the edge of the firelight, deciding.
What keeps it from being simple revelry is that the song knows it is temporary. The dancers get "Einmal nur im großen Kreise" (only once in the great circle), and the night ends on a clock: "Bis das erste Morgenlicht / Unser Traumgewebe bricht" (until the first morning light breaks our web of dreams). Even the good and evil words of the sixth line are carried along together rather than sorted out — this is a night off from the distinction. The long instrumental stretch in the middle is the dance itself, and when the fiddles come back at the end the words have narrowed to just the chorus, the way a refrain survives when everything else has been danced away.