Question: When did it become a tradition to stand for the national anthem?
Disambiguated questions and answers: question: When did it become a tradition to stand at attention for the national anthem?
When did it become a tradition to stand at attention and face in the direction of the music for the national anthem?
When did it become a tradition to stand for the national anthem, with hand over the heart and removing hat?
answer: June 22, 1942
December 23, 1942
July 7, 1976
Natural Questions answer: 1942 (from the Wikipedia article "The Star-Spangled Banner")