In 1952, President Harry S. Truman signed a bill proclaiming a National Day of Prayer must be declared by each subsequent president at an appropriate date of his choice. In 1988, the law was amended so that the National Day of Prayer would be held on the first Thursday of May.
Wikipedia contributors, "National Day of Prayer," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=National_Day_of_Prayer&oldid=947485258 (accessed May 6, 2020).