Thanksgiving

The story of the Pilgrims and Indians shared every year in November leaves out something that I want to elaborate on, the date. Why do we celebrate Thanksgiving in late November? Is that when the Pilgrims and Indians had their first feast? No one knows the date of that first feast of Thanksgiving. It was probably in the early 1600 but that is only speculation. So how did we come to the designated time we now celebrate as Thanksgiving Day?
December 18th 1777 was proclaimed by our first President, George Washington, as a day of Thanksgiving to thank the Almighty God for the military victory over the British. Each year there was a different day proclaimed as the Day of Thanksgiving to the Almighty God for a various reasons, depending on the current events. None of these were held in November until 1863 when the forth Thursday of November was observed as Thanksgiving Day and continued until 1939. F.D. Roosevelt was President and he changed the date to the third Thursday of November for two consecutive years then made it a part of Federal Law that Thanksgiving Day would be held on the next-to-last Thursday of November.
If we can accept this as our National Thanksgiving Day, knowing that it is not the actual date of the Pilgrims first feast with the Native Americans, why can't we accept December 25th as the day of celebrating the birth of Jesus without having problems?