eve of the main festival; small festival on the night before the main festival
Especially associated with the Gion Festival in Kyoto, where the streets are closed to traffic and lined with stalls on the nights leading up to the main parade.
See also: 宵宮
Many tourists visit the Gion Festival's Yoiyama every year.
On the night of Yoiyama, I look forward to walking around the stalls in a yukata.
Compound of 宵 (yoi, 'evening') and 山 (yama, 'mountain'), referring to the decorated floats (山鉾) that are lit up and displayed on the eve of the festival.