In the spring, on Mayday, Morris dancers "dance the sun up" lest it fail to warm the land for the coming summer. This custom is widely observed across the Multiverse (even on worlds where the Carbon Dioxide freezes in Winter) and especially on the Discworld, where the threat of stories and legends is real and such rites are more than just quaint old customs. Mayday is a festive occasion and many will drag themselves out of bed before dawn to watch and cheer on the dancers and the sun. Later in the day, perhaps after a nap, there will be more dancing, Morris as well as the maypole, with feasting and frolic and beer.
On or near the first of Ember there is another dance, the last of the year. There is no crowd of cheering spectators, no merriment, no music, no sound. The dancers trade their bright white beribboned costumes for black shrouds and perform in darkness and silence without bells, sticks or swords (or better, with bells of octiron that create a lacy counter-tinkle of silences). This is the Dark Morris, without which winter may not arrive to complete the cycle, water the land with snow and prepare for the renewal of spring. Winter begets summer, death begets life, and a city-sized sun that travels faster than its own light needs all the help it can get.
Tiffany Aching's unfortunate decision to join in the Dark Morris precipitated the events of Wintersmith when she found herself in the wrong place at the wrong time, inspiring contrasting emotions in the Wintersmith and the Summer Lady.
The Dark Morris has since become a real performance piece at folk festivals in Roundworld. When performed at festivals it is typically set to music, but at the end of Wintersmith Pratchett relates a story of a side of Morris Men who performed the dance in total silence for him at a book signing.