Flag Day itself—the anniversary of the official adoption of the U.S. Flag in 1777—had been celebrated in an unofficial capacity on and off long before 1950. Woodrow Wilson may have started the modern observance of the holiday in his 1916 proclamation, in which he called it "a day upon which we should direct our minds… to thoughts of the ideals and principles of which we have sought to make our great Government the embodiment." During World War II in particular, Flag Day took on a new significance. Franklin Delano Roosevelt, in his Flag Day proclamation of 1944, called upon Americans to "display our flag proudly, knowing that it symbolizes the strong and constructive ideals—the democratic ideals—which we oppose to the evil of our enemies. President Truman expressed similar sentiments in his 1945 proclamation of the holiday: "As we press forward to final victory, we are strengthened with the knowledge that for millions of people in other lands as well; as in our own our Flag is a living token of human integrity and freedom."
Despite its de facto acceptance as a holiday, however, Flag Day was still ultimately unofficial, proclaimed only by the President and thus subject to the whims of each individual community. In the summer session of the 1949 Congress, however, the House and the Senate agreed to H.J. Res. 170, a joint resolution officially recognizing June 14 of each year as Flag Day and "authoriz[ing] and request[ing]" the president to issue an annual proclamation informing the American people of the occasion. By throwing their support behind the holiday, Congress legitimated it and made it a truly national holiday. Truman made his first announcement of Flag Day as approved by Congress on May 27, 1950.
Harry Truman was particularly fond of Flag Day, in part because on June 14, 1905, he enlisted in the Missouri National Guard, an experience which would be of little significance at first but would change his life more than a decade later when he went to serve in the First World War. Truman also presided over the 175th anniversary of Flag Day in 1952, and in his proclamation of that year, in the midst of the Korean War, he reminded the public that the freedom the flag stood for had two elements: "I also urge all our citizens to give special thought on Flag Day not only to their many rights and privileges but also to their duties and responsibilities under the national ensign…".