Short answer
Mid-October to early November. The forest around the lakes turns yellow, orange and red, the water is still full, and the travertine lakebeds throw the colour back up. This is also the busiest two weeks of the year, and the park's daily cap sells out — book the moment tickets are released.
Season by season
Spring (April–May). The park reopens after winter and the water is low but clear. Snow can still fall, some boardwalks stay closed, and the forest is bare until late April. Quiet, cheap, and not the Jiuzhaigou of the photographs.
Early summer (June). Water levels recover, the forest greens up, and the crowds are still moderate. This is the most underrated month: the lakes are full, the light is good, and you can walk without a queue.
Summer (July–August). Rainy season. The park is lush and the waterfalls are at their loudest, but days are often overcast and afternoon rain is likely. The compensation is that the greens are extraordinary and the tour groups are spread out.
Autumn (September–November). September is green with early colour on the higher slopes; October is the peak, with the classic red-and-gold forest reflected in the lakes; November is quieter and colder, with the last colour fading by mid-month. If you can only come once, come in the second or third week of October.
Winter (December–March). Cold, quiet, and partly closed. The waterfalls freeze into blue-white columns, the lakes take on a deep green, and the park runs a reduced bus service with some trails shut. If you want Nuorilang and Long Lake without another person in the frame, this is when to come — but check what is open before you book.
What the season changes in practice
Water level is the real variable. In winter and early spring, some lakes shrink and the travertine shoals become visible; in late summer they are full and the colour is deeper. The park's shuttle service and the opening hours of the upper valleys also vary by season, so a winter visit needs a phone call or a check of the official account before you commit.