Fall Ingathering
In the human life cycle, autumn is a time of reflection, review, and assessment of life already lived. For animals, it's preparation for the winter: think of squirrels stuffing their cheeks with nuts to hide, field mice beginning to seek out places for winter warmth.
For us gardeners, it is when we bring in the last of the summer garden. We dig sweet potatoes, dry and grind corn, put up leather britches (dried beans on a string), can, freeze, and dry as if our winter bellies depended on it. We look at how many weeks or months of winter we might have and like the cheeky squirrels, work to make sure we've put enough by.
The classic Celtic Samhain became our Hallowe'en when carved pumpkins show plenty. The Korean Choosuk is a time of remembering one's ancestors, and give thanks, West Africa has Yam Festivals, There are Japanese harvest festivals, Turkish, Chinese, Vietnamese, Russian indigenous, Southern Indian, and Italian harvest festivals.
Around the world, as key crops are brought in, people give thanks in many different ways--many of which include honoring ancestors and thanking the gods of fertility and good fortune who ensured the harvest.
Whatever rules, ideas, ceremonies, and traditions are common to your people, the culture of thanksgiving is more than a day in November that leaves turkeys in fear for their lives. Harvest requires that thanks be given for the fruit the land yields on your behalf.