Thanksgiving roots are not in 1621, Plymouth as you might have been accustomed to think.
The Day of Thanksgiving was originally conceived in England in the English Reformation during the reign of Henry 8th. Prior to the reformation there were 95 church holidays and 52 sundays when people were required to attend church and forego work. 95 church holidays! That is not a typo.The priorities during that era were certainly very different from today.
The 1536 reforms reduced the number of holidays to just 27! Some wanted to eliminate even these holidays and replace them with Days of Thankgiving and Days of Fasting. Positive events such as winning a great war, the birth of a royal heir and so forth would be followed with a day of Thanksgiving whereas terrible crisis such as drought, plague or floods would be followed by days of fasting. Great Negative and Positive events were given divine significance that either required pennance to ensure the continued Blessings of the Divine or Thanks to the Divine.
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