One of the Big Tohoku Summer Festivals, Yamagata Hanagasa Matsuri is an evening folk dance parade of women and children wearing colorful costumes and waving flower hats. “Hanagasa” means flower hat. Normally held on three evenings from Aug. 5 to 7 on the city’s main road near JR Yamagata Station (upper left photo).
Over 14,000 dancers from about 120 troupes of all ages appear during the three evenings (almost 5,000 dancers/evening) from 6 pm to 9:30 pm. They dance to the beat of taiko drums and the prerecorded folk tune of “Hanagasa Ondo“ (花笠音頭) chanting, “Yassho, makasho!” An average of 300,000 tourists see the festival per evening.
The dance originated as a rice-planting prayer for a good harvest, and the festival started in 1963. Photogenic parade, and it’s not so crowded either. Many dance troupes are sponsored by major companies, so the company name or logo is prominently emblazoned on their costumes. It really is a long advertising parade. Free admission.
Due to the pandemic, the main dance parade was canceled in 2020 except for a few mini dances inside JR Yamagata Station for PR (bottom left photo). In 2021, the festival will be held (yay!), but not on the public road. It will be held only on two evenings Aug. 5 and 6, 2021 at Kirayaka Stadium. They will likely just dance around the field and the crowd will be kept in the stands. Possible to see the festival as a day trip from Sendai (Miyagi), but staying overnight is fine too. 
Nearest station is JR Yamagata Station. From Tokyo, it takes almost 3 hours via Yamagata Shinkansen.
Official: http://www.hanagasa.jp/
Video: https://youtu.be/Tu4X7ATHVOk