Why is the ninth month called September?
c.1300, "handful or bundle of hay, grass, etc.," used for burning or cleaning or as a cushion; perhaps from an unrecorded Old English word, cognate with Norwegian and Swedish visp "wisp," of unknown origin; sometimes said to be connected with whisk or with Middle Low German and Middle Dutch wispel "a measure of grain." Meaning "thin, filmy portion" first attested 1836.
["An Experiment with a Self-Compiling Compiler for a Simple List-Processing Language", M.V. Wilkes, Ann Rev Automatic Programming 4:1-48. (1964)].