April 30
The Celtic people divided the year in two seasons. Winter started November 1 (Samhain / Haloween) and Summer May 1 (Beltaine). The night before they lighted purification fires on mountains and hills.
A similar tradition existed among Germanic and Scandinavian people, the Walpurgis Night (Walpurgisnacht, Valborgsmässafton...). The name comes from Saint Walburga, active in Germany and declared a saint May 1, 779. She was by the Germans honored in the same way as the Vikings celbrated the arrival of spring, by fires. This was later somehow mixed together.
To this you can add the tradition about the witches, again of German origin.
In the Nordic countries, other traditions are also linked to April 30, often related to the student life (carnivals, singing...). How long will these traditions last?
There is, to my knowledge, no specific April 30 celebration around Paris, so I had to steal a picture.