The National Day of Prayer is an annual day of observance designated by the
United States congress, when people are asked "to turn to God in prayer and
meditation". On April 17, 1952, President Harry S. Truman (below) signed a bill proclaiming a National Day of Prayer must be declared by each following U.S. 
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.

President Harry S. Truman proclaiming a National Day of Prayer