In the United States the last Thursday in November is always observed as Thanksgiving Day. Ostensibly, it commemorates a 1621 event celebrating a good harvest shared by the new colonists arrived from Plymouth, England( now called “Pilgrims”) who came to these shores seeking religious and economic freedom and the Native American Wampanoags who lived here already. The observance as it exists now in the US and its Territories dates as far back as 1863 but having it fixed on the fourth Thursday in November occurred in 1941 under President Franklin Roosevelt. From 1939 to 1941 it had been fixed on the next to last Thursday in November for business reasons but a joint resolution in Congress changed it to the fourth Thursday in November where it has remained since.
Some armorial devices that are somewhat related to Thanksgiving include:
EXARANDORUM 
The seal of the Plymouth Colony was the cross of St George amidst four “Praying Indians” – converts to Christianity. The armorial dish contains some questionable arms. Those of Standish and Winslow are genuine. The others, while genuine arms, may have been assumed by 19th century descendants of the Pilgrims simply because they shared the same surname as the original armiger. Miles Standish claimed kinship with the ancient Catholic Lancashire family of Standish and may have been baptized a Catholic, but he certainly left the Church by the time of his adulthood. One surprising Plymouth Catholic was Squanto who was taught and converted by the Spanish Franciscan friars in Malaga, who bought him out of slavery, before his return to New England to help the Pilgrims. A Catholic priest visited Plymouth in 1650: Gabriel Druillettes SJ. Governor Bradford gave him a private house so he could say Mass and made sure he had fish on Friday. Although they are often confused with them, the Pilgrims were not as intolerant as their Boston neighbors, the Puritans. Geoffrey Gamble, Governor of the Delaware Society of Mayflower Descendants.