JFK's 1963 Thanksgiving Day Proclamation
(He signed this 11.4, but it was published 11.5, and Thanksgiving that year was 11.28.)

Here's a portion:

"Much time has passed since the first colonists came to rocky shores and dark forests of an unknown continent, much time since President Washington led a young people into the experience of nationhood, much time since President Lincoln saw the American nation through the ordeal of fraternal war...

Yet, as our power has grown, so has our peril. Today we give our thanks, most of all, for the ideals of honor and faith we inherit from our forefathers--for the decency of purpose, steadfastness of resolve and strength of will, for the courage and the humility, which they possessed and which we must seek every day to emulate. As we express our gratitude, we must never forget that the highest appreciation is not to utter words but to live by them."

https://www.presidency.ucsb.edu/documents/proclamation-3560-thanksgiving-day-1963