eve of a festival; small festival held the night before the main festival
Refers to the festivities on the evening before the main festival day (本祭り). Often involves smaller-scale events, lanterns, and food stalls.
See also: 本祭
At the eve of the festival, locals set up food stalls and it gets lively.
Since the main festival is tomorrow, let's enjoy the eve of the festival tonight.
Standard spelling with okurigana.
Abbreviated kanji spelling without okurigana; less common but still used.
Compound of 宵 (よい, 'evening') and 祭り (まつり, 'festival'). The exact historical derivation is uncertain, but the term naturally describes the pre-festival evening events.