sometimes autum can be a name sometimes it is not a name
If you meant autumnal, that's an adjective that means, "belonging to or suggestive of autumn; produced or gathered in autumn or past maturity or middle life." Please see link below.
In the northern hemisphere it is the 21st of September. In the southern hemisphere it would be the 21st of March.
The equinox in autumn, on about September 22 in the northern hemisphere and March 20 in the southern hemisphere.