Here is the U.S. law related to the observance of Memorial Day.

36 U.S. Code § 116. Memorial Day

(a) Designation — The last Monday in May is Memorial Day.

(b) Proclamation — The President is requested to issue each year a proclamation—

  (1) calling on the people of the United States to observe Memorial Day by praying, according to their individual religious faith, for permanent peace;
  (2) designating a period of time on Memorial Day during which the people may unite in prayer for a permanent peace;
  (3) calling on the people of the United States to unite in prayer at that time; and
  (4) calling on the media to join in observing Memorial Day and the period of prayer.

You can read this year's presidential proclamation here. In it, President Trump designates "the hour beginning in each locality at 11:00 a.m. [today] as a time when people might unite in prayer."

Further, all Americans are requested "to observe the National Moment of Remembrance beginning at 3:00 p.m. local time."

O beautiful for heroes proved
In liberating strife,
Who more than self their country loved
And mercy more than life!

America! America!
May God thy gold refine,
Till all success be nobleness,
And every gain divine!

– from "America the Beautiful' by Katharine Lee Bates