When is the harvest season for walnuts?

The harvesting season depends upon the variety and region walnuts are grown in. Walnut tree harvesting starts from early September to early November. At this point, the kernels are light in color and the membrane between the halves has turned brown. The harvest begins when the nuts have matured and a high percentage of nuts on the tree have split hulls, usually, a small number of nuts have already started to naturally fall to the ground. In that case, collect nuts as soon as possible to avoid mold. Hulls soften naturally over time, allowing easy access to the nut.

Sources:

Walnut Harvesting & Processing

How to harvest, process and store black walnuts

Walnut Tree Harvesting: When Are Walnuts Ready To Pick