Why is the ninth month called September?
1859 as the years of someone's life between 70 and 79; from 1837 as the eighth decade of years in a given century. See seventy.
Old English (hund)seofontig, from seofon (see seven) + -tig (see -ty (1)). Cf. Old Frisian soventich, Middle Dutch seventich, Old Norse sjau tiger.