In early use: the night before Saturday, Friday night; (also as adverb, with the): on the night before Saturday; on Friday night (obsolete). In later use: Saturday night (informal).
Origin
Old English. From Satur- + night. Compare Sunnight, Tuesnight, Wednesnight, Frinight, and also Thurseven.
Definition of Saturnight in US English:
Saturnight
nounˌsatəˈnʌɪt
In early use: the night before Saturday, Friday night; (also as adverb, with the): on the night before Saturday; on Friday night (obsolete). In later use: Saturday night (informal).
Origin
Old English. From Satur- + night. Compare Sunnight, Tuesnight, Wednesnight, Frinight, and also Thurseven.