The interfaith Thanksgiving service was on its last legs more than a decade ago when I was the pastor at Universalist National Memorial Church (UNMC), in Washington, D.C. It was one of those liberal Jewish-Protestant events that was far more common two generations before, but it's hard to maintain a tradition when that was your sole surviving point of contact.
I hadn't thought about it in years when, a few months ago while studying the Classical Reform Jewish tradition I ran across a reference to it in the persan of Washington Hebrew Congregation's (WHC) then-senior rabbi Norman Gerstenfeld. WHC was traditionally a participant in the Thanksgiving service with UNMC.
Even in 1963. Six days after President Kennedy was killed, the nation celebrated Thanksgiving.
President Johnston attended the morning interfaith Thanksgiving service at Mount Vernon Place United Methodist Church, which included UNMC (then pastored by the long-tenured Seth Brooks) and WHC.
The other congregations taking place were Congregation Adas Israel, Calvary Baptist Church, and National City Christian Church (Disciples of Christ; President Johnston's church). All are extant.
For reportage, and notes of Johnston's participation, see
"Johnson's Thanksgiving Address Asks Nation to 'Banish Rancor' and Move On to 'New Greatness'" by Tom Wicker (New York Times)