The National Day of Prayer is an annual day of observance held on the first Thursday of May, designated by the United States Congress, when people are asked "to turn to God in prayer and meditation". The president is required by law (36 U.S.C. § 119) to sign a proclamation each year, encouraging all Americans to pray on this day.

The modern law formalizing its annual observance was enacted in 1952 as part of the public reaction to the threats perceived in the Korean War, although earlier days of fasting and prayer had been established by the Second Continental Congress from 1775 until 1783, and by President John Adams in 1798 and 1799. Thomas Jefferson established a day of prayer and thanksgiving, but this occurred while he served as governor of Virginia.

The constitutionality of the National Day of Prayer was unsuccessfully challenged in court by the Freedom From Religion Foundation after their attempt was unanimously dismissed by a panel of a federal appellate court in April 2011; the Alliance Defense Fund (now known as the Alliance Defending Freedom) provided the defense for observance of the National Day of Prayer.