The National Day of Prayer in the United States is designated as the first Thursday in May of each calendar year. It is open to people of all faiths.
While days of prayer were common in the history of the United States, the current bill was signed into law by President Harry Truman on April 17th, 1952. In 1988 President Ronald Reagan set the current date of the first Thursday in May.
The National Day of Prayer Task Force is a Christian organization with ties to Focus on the Family that helps to mobilize prayer on the National Day of Prayer. While its focus is on energizing Christians, the encouragment to prayer is for all faiths.