On the 15th of February, 1631, a ship named the Lyon landed in Nantasket, in Massachusetts Bay. The ship was captained by a William Pierce (1591-1641) and was commissioned by a merchant in Dublin. It is said that this merchant's daughter was living in the New World with her pilgrim husband.
The winter of 1630 had been a particulary harsh one in Plymouth, and the people were all but starved, surviving on a meagre diet of whatever shellfish and nuts they could forage. When the Lyon arrived, it carried with it 20 passengers and a large amount of provisions. So bountiful was the cargo, that the people of Plymouth redesignated the 20th of February from a day of prayer and fasting to one of gratitude and feasting.
Historians are divided on whether or not this is in fact the true origin of Thanksgiving as we know it today, but it is entirely possible that the saving grace of the Lyon's shipment was what allowed the first settlers to persue their lives on the frontier.
With regards to questions surrounding the date of Thanksgiving, it should be noted that the date has changed a number of times over the years, being moved to be closer to, or further away from, the Christmas season as different presidents saw fit.