A small party or gathering of people that drink from 40 oz. of beer instead of cases and kegs and listen to music. It is the party goers' choice to drink from 40's, not because of lack of money; however, that can also be the case.
Common to some of suburbia.
It is not uncommon to see couples slow dancing during the later hours of these gatherings.
Variations of the term: 40-Night, 40 Night, and 40NA.
Though not exclusive to Sunday nights, that is when this type of party originated.