I live and teach in apple country, the Yakima Valley. Apples are harvested from Mid-September through Mid-October usually, so the poem "After Apple Picking" likely takes place in mid to late October, after the last varieties have been picked.
Another way we can tell this is by the line about "Essence of Winter sleep is on the night", which suggests that the air is cold, as in late October, maybe even early November. He also speaks of apples hitting the ground, and being turned into cider, the byproduct of the end of harvest.
In the poem "After Apple Picking" the poet Robert Frost had started out by being so proud of his beautiful apples and his productive orchard and looking forward to the season for picking them - the harvest as a just reward for al his stewardship of the fruit during the year. But every time I read this wonderful poem, I always wonder whether he left the picking until it was almost too late - Fall as it turns to Winter. At that time, water might be frozen on the top in the trough, mornings would be frosty and some creatures starting to think about hibernating. Of course, the apples need to be gotten in very fast then, before the fruits are spoiled by cold, winds or ice. I think he misjudged the length and onerous nature of the task, so that in hurrying he lost the joy of it and could only feel the pain of rungs under his feet and sore muscles when he finally got to bed.
In the poem "After the Apple Picking" it is the end of the harvest season. It has not been over long because the man is still feeling the steps of the ladder rung under his feet. Apples are harvested in the fall usually until mid October in some regions. I believe that the season is late fall around the beginning to middle of October.
Robert Frost grew up in New England and the poem takes place there. This supports the harvest time even more. Apples on the ground have rotted and the woodchuck has begun to hibernate in preparation for the winter months.