Each year from around mid to late October, Gunma's Koizumi Cosmos Fields bloom with millions of colorful flowers in a space of almost 11 hectares. Part of the appeal of this event is that the beautiful cosmos flowers sit right by a large torii arch from the nearby Koizumi Inari Shrine, creating a wonderfully photogenic scene.
Do note that like all flower events, the dates can shift slightly depending on the blooming conditions. It's advisable to check the official event website before attending so you can be informed of any updates.
Admission to the cosmos fields is free of charge.