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 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,
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.

John Adams (October 30, 1735 – July 4, 1826)

On May 7, 1915, less than a year after World War I (1914-18) erupted across
Europe, a German U-boat torpedoed and sank the RMS Lusitania, a British
ocean liner en-route from New York to Liverpool, England.

The earlier German attacks on merchant ships off the south coast of Ireland prompted the British Admiralty to warn the Lusitania to avoid the area or
take simple evasive action, such as zigzagging to confuse U-boats plotting
the vessel’s course. The captain of the Lusitania ignored these suggestions,
and at 2:12 p.m. on May 7, in the waters of the Celtic Sea, the 32,000-ton
ship was hit by an exploding torpedo on its starboard side. The torpedo
blast was followed by a larger explosion, probably of the ship’s boilers. It
sank within 20 minutes.

Sinking of the Lusitania by John Keay.