DID YOU KNOW The National Day of Prayer shares common roots with the celebration of Thanksgiving?
It does in that BOTH were national proclamations establishing a day of prayer.

In June of 1774 the Colony of Virginia's House of Burgesses established a day of fasting and prayer, to protest the Boston Port Act, such that the people of Virginia would assemble for prayer led by clergymen. This is what Thomas Jefferson wrote:
"The effect of the day through the whole colony was like a shock of electricity", moving the Virginians to choose delegates to establish self-rule."

In July of 1775, the Continental Congress issued a proclamation to ALL the "English Colonists" recommending, "A day of public humiliation, fasting, and prayer" for a resumption of "the just rights and privileges of the Colonies" in "civil and religious" matters.  John Adams wrote this about that prayer time: "The popular response was gratifying, that the special day was more widely observed than the practice of going to church on Sunday."

After this success, Congress determined to call for a day of PRAYER and FASTING each SPRING. and a day of PRAYER and THANKSGIVING each FALL.

Besides these 2 observances
The call to prayer for decisions and times of war through out our history is phenomenal.

However, after James Madison, there was a period of 47 years, from 1815 to 1862, where there was no presidential prayer  None of the next eleven presidents issued prayer proclamations.

But Forward 90 years

In January–February 1952 during the Korean War,  Reverend Billy Graham, stated the desire of United National Prayer. "What a thrilling, glorious thing it would be to see the leaders of our country today kneeling before Almighty God in prayer. What a thrill would sweep this country. What renewed hope and courage would grip the Americans at this hour of peril."

Representative Percy Priest from Tennessee observed that Graham had issued a challenge for a national day of prayer.
Members of the House and Senate introduced a joint resolution for an annual National Day of Prayer,
"on which the people of the United States may turn to God in prayer and meditation at churches, in groups, and as individuals."

On April of that same year, 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, 36 yrs later, the law was amended by George H. W. Bush so that the National Day of Prayer would be held on the first Thursday of May.

NATIONAL DAY OF PRAYER TASK FORCE
14 yrs later, In 2002, The National Day of Prayer committee started as a subcommittee.  Five years later,  they officially formed as a  Task Force. The National Day of Prayer Task Force MISSION states: "Centered on the Lord Jesus Christ, the National Day of Prayer Task Force exists to mobilize UNIFIED PUBIC and PERSONAL prayer for America."