Crueltide is a confusing concept which seems to have been forgotten since the early books. It was described as the end of the half-year, alternating with Hogswatch as the coldest, darkest point in the Ankh-Morpork Calendar year. Latterly, Hogswatch appears to occur every University Calendar year and Crueltide isn't mentioned. The Discworld Almanak suggests a Hogswatch every December and seems silent on Crueltide.
Suggest possible explanations in discussion.