“Because of the faith of many of our founding fathers, public prayer and national days of prayer have a long-standing and significant history in American tradition. The Supreme Court affirmed the right of state legislatures to open their sessions with prayer in Marsh vs. Chambers (1983).
“The National Day of Prayer is a vital part of our heritage. Since the first call to prayer in 1775, when the Continental Congress asked the colonies to pray for wisdom in forming a nation, the call to prayer has continued through our history, including President Lincoln’s proclamation of a day of “humiliation, fasting, and prayer” in 1863. In 1952, a joint resolution by Congress, signed by President Truman, declared an annual national day of prayer. In 1988, the law was amended and signed by President Reagan, permanently setting the day as the first Thursday of every May. Each year, the president signs a proclamation, encouraging all Americans to pray on this day. Last year, all 50 state governors plus the governors of several U.S. territories signed similar proclamations. ”
Taken from http://www.nationaldayofprayer.org/about