Is the autumnal equinox longest day?

No, the autumnal equinox is not the longest day. The autumnal equinox is the day that the Sun crosses the celestial equator, moving from the Northern Hemisphere to the Southern Hemisphere. This occurs twice a year, around September 22nd and March 20th. The longest day of the year is the summer solstice, which occurs around June 21st in the Northern Hemisphere and December 21st in the Southern Hemisphere.