Why is the ninth month called September?
late 14c., from Latin aequalis "uniform, identical, equal," from aequus "level, even, just," of unknown origin. Parallel formation egal (from Old French egal) was in use late 14c.-17c. The noun is recorded from 1570s.
1580s, "compare, liken," also "match, rival," from equal (adj.). Related: Equaled; equaling.