From Longman Dictionary of Contemporary Englishopen dayˈopen day noun [countable] British English SSHOW/LET somebody SEE somethinga day when a school or an organization invites the public to come in and see the work that is done there
Examples from the Corpus
open day• Opening success: More than 1000 people visited Middlesbrough botanic centre's open day.• Members of the public took the opportunity to see for themselves in a unique open day. 