The Pilgrims did not hold a true Thanksgiving until 1623, after a switch from communal farming to privatized farming finally resulted in a larger harvest. Irregular Thanksgivings continued after favorable events and days of fasting after unfavorable ones. In the Plymouth tradition, a thanksgiving day was a church observance, rather than a feast day.
Gradually, an annual Thanksgiving after the harvest developed in the mid-17th century. This did not occur on any set day or necessarily on the same day in different colonies in America.