Final answer:
True, the first national Thanksgiving Day was established by President George Washington and a congressional resolution on November 26, 1789, to give thanks for the new Constitution, separate from the Pilgrims' 1621 feast.
Step-by-step explanation:
The statement that President George Washington and a congressional resolution established the first national Thanksgiving Day on November 26, 1789, to give "thanks" for the new Constitution is True. The first national Thanksgiving was indeed proclaimed by Washington after a resolution by Congress. However, it's important to differentiate that this Thanksgiving was not the same as the famous "first" Thanksgiving celebrated at Plymouth by the Pilgrims and Native Americans in 1621. While this earlier event is often cited as the origin of the holiday, the establishment of a national day of Thanksgiving was to recognize the ratification of the Constitution and the new government's establishment.