noun
main festival; main day of a festival
Refers to the primary festival event or the most important day within a multi-day festival, often contrasted with 宵祭り (eve festival) or 陰祭り (alternate/smaller festival).
本祭は明日行われます。
The main festival will be held tomorrow.
宵祭りも楽しいけど、やっぱり本祭が一番盛り上
The eve festival is fun too, but the main festival is definitely the most exciting.
Refers to the eve or preliminary festival held the night before the main festival (本祭).
Refers to a smaller, alternate, or less prominent festival held in place of or alongside the main festival.
Compound of 本 (hon, 'main') + 祭 (matsuri, 'festival'). The reading ほんまつり follows standard on'yomi for 本 and kun'yomi for 祭, a common pattern in festival terminology.