Why is the ninth month called September?
1620s, a cry to call attention from a distance, a variant of who. As a command to stop a horse, it is attested from 1843, a variant of ho. As an expression of delight or surprise (1980s) it has gradually superseded wow, which was very popular 1960s.
[fr whoa! ''Stop, horse!'' attested fr the 1840s]