It is also known as LUGHNASADH. From WitchVox:

The First Harvest. Plants are 'setting their seed" already for the next year as the cycle of Nature continues. The Sun (Son) still burns brightly, but already the passing of the days begins to herald the coming of Autumn. The young animals are now almost full grown and our plans-planted with high expectations in the spring- are beginning to come to fruition.