What was first marked on 26 November 1789 in the new USA as a nation-wide celebration?

Question

11 November is marked in many countries as Remembrance Day, Poppy Day, Armistice Day or Veterans Day to commemorate the sacrifices of members of the armed forces and civilians in time of war. Why has that date been selected?

Answer 1

Thanksgiving Day
An annual Thanksgiving Day had been observed before this in several colonies but this was the first to be observed nation-wide in the USA, declared by George Washington.