As the name of the season suggest, it will adapt the manga's Entertainment District Arc, which covers chapters 70-99 of the manga. In the arc, Tanjiro, Inosuke, and Zenitsu accompany the Sound Hashira, Tegen, on a mission to Yoshiwara to help search for his missing wives who were investigating rumors of demons within the district.
Kimetsu no Yaiba: Entertainment District Arc is set to premiere in 2021.
[Source: Anime News Network, arc summary from Demon Slayer Wiki]