It's fun to imagine what the day was like for the Pilgrims and Native Americans during the first Thanksgiving. There are numerous historical accounts that paint a broad picture of the clothes they must have worn and the foods they might have eaten. There are even clues as to the entertainments they may have had. No, there was not football at the end of a huge turkey dinner but they likely did have games and song and story telling after a feast of fish and deer and small game.
However the historical record is lacking in one aspect of that first three day Thanksgiving. The weather. Perhaps David Ludlum has provided the most amazing chronicle of the earth's historical weather of any writer. I heard him speak once and I can tell you that the number of obscure facts he retained was truly flabbergasting. In his wonderful book Early American Winters he scours written accounts of the times to fashion a good guess as to the weather report for the first Thanksgiving.
There was much written about the brutal weather that greeted the Pilgrims arrival in Massachusetts. Frigid Winter storms kept the settlers on their boat for weeks before the weather broke. Account hold that the winter of 1620-21 ended mild after the hard start. There were several written accounts of the snow depth and frozen soil and windy conditions.
However little was written about the weather on the days of Thanksgiving. Edward Winslow, a Separatist who traveled on the Mayflower did write that the winter was characterized by "remarkable mildness". But it was the lack of notation regarding the weather that suggests that it was uneventful.
The first Thanksgiving was likely held in the first weeks of October, which is climatologically a comfortable time in New England. It would be over 100 years later that George Washington would change the official date to November. So with the clues given and the historical weather records it would be a reasonable guess that the first Thanksgiving was not too hot, or too cold, or too rainy, or too sunny, or too cloudy but rather just average and probably in the 60's.