Spring — particularly April through June — is the best time for the Qiandongnan region. The rice terraces around the villages are planted and lush, the weather is mild at around 15–22°C, and a number of major Miao festivals fall in this season. The Miao Sister Festival (Zimei Jie) at Taijiang, usually in April or May of the lunar calendar, is the largest — a multi-day courting ceremony involving traditional dress, lusheng music, and communal meals, drawing participants from dozens of surrounding villages.
Autumn (September–November) is also excellent: clear skies, harvest landscapes of golden rice, and fewer visitors than the spring festival season. Summer is hot and wet; the villages are photogenic in the mist but the humidity is challenging. Winter is cold in the mountains and some village guesthouses close for the season.
If a specific festival is your reason for visiting, check the lunar calendar carefully — the dates shift each year relative to the Gregorian calendar. The Kaili Tourism Bureau website (in Chinese) posts festival calendars annually.