noun
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: 本祭り
宵祭りでは、地元の人たちが屋台を出して賑わう。
At the eve of the festival, locals set up food stalls and it gets lively.
明日が本祭りなので、今
Since the main festival is tomorrow, let's enjoy the eve of the festival tonight.
本祭り is the main festival day, while 宵祭り is the smaller celebration held the night before.
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.