Why is the ninth month called September?
1660s, "pertaining to the millennium," from stem of millennium + -al (1). Meaning "pertaining to a period of 1,000 years" is from 1807. As a noun from 1896, originally "thousandth anniversary." From 1992 as a generational name for those born in the mid-1980s and thus coming of age around the year 2000.