Why is the ninth month called September?
late 12c., "time when something begins," from begin. Meaning "act of starting something" is from early 13c. The Old English word was fruma.
Old English beginnan "to begin, attempt, undertake," a rare word beside the more usual form onginnan (class III strong verb; past tense ongann, past participle ongunnen); from bi- (see be-) + West Germanbic *ginnan, of obscure meaning and found only in compounds, perhaps "to open, open up" (cf. Old High German in-ginnan "to cut open, open up," also "begin, undertake"), with sense evolution from "open" to "begin." Cognates elsewhere in Germanic include Old Frisian biginna "to begin," Middle Dutch beghinnen, Old High German beginnan, German beginnen, Old Frisian bijenna "to begin," Gothic duginnan.