Walpurgis Night is generally celebrated on April 30, just prior to May Day in England at Northern Europe with bonfires, singing, and ceremony. It is meant to be the six month polar opposite of Halloween in Wiccan culture.
Widdershins, the title, is an old term (earliest known usage in 1503) meaning counterclockwise. Originally from the Middle Low German "weddersinnes" (literal meaning: "against the way") its opposite is "deosil", meaning clockwise or with the sun. Used in some modern Pagan/Wiccan practice.
In one of the scenes, Robert mentions and shows a photograph of Uncle Albert receiving a cup winners medal from King George VI in 1948. This, although never mentioned, suggests that Uncle Albert played for Manchester United, who won the cup that year beating Blackpool 4-2.