jubilee; semicentennial; 50th anniversary celebration
Refers to a celebration or ceremony marking the 50th year of an event, institution, or reign. Often used in formal or historical contexts.
This year, the city's 50th anniversary celebration will be held.
I attended the commemorative ceremony for the 50th anniversary.
記念日 is a general term for an anniversary or memorial day, while 五十年祭 specifically denotes a 50th anniversary celebration or festival.
周年祭 is a broader term for an anniversary festival of any number of years; 五十年祭 specifies the 50th year.
Compound of 五十 (fifty), 年 (year), and 祭 (festival/celebration). The reading ごじゅうねんさい follows standard on'yomi.