Proper noun
(UK, Canada, US) A particular night during which young people play pranks and do mischief in their neighborhoods.
(dated, originally) The night before May Day.
(UK) The night before Bonfire Night.
(US, Maryland, Delaware, Pennsylvania, New York, Massachusetts, Rhode Island, Connecticut, and especially New Jersey) The night of the 30th to the 31st of October (the night before Halloween).
Source: en.wiktionary.org