| National Physical Fitness and Sports Month | May |
| National Youth Traffic Safety Month | May |
| Sudoku First Published as Number Place in Dell Puzzle Magazine | May, 1979 |
| May Day | May 1, 2010 |
| Mother Goose Day | May 1, 2010 |
| Whale Awareness Day (in Massachusetts) | May 1, 2010 |
| Space Day | May 2, 2010 |
Elizabeth Jane Cochran's, Known as Nellie Bly, Birthday | May 5, 1864 |
| Cinco de Mayo | May 5, 2010 |
| Cartoonist Day | May 5, 2010 |
| Eta Aquarids Meteor Shower | May 5-6, 2010 |
| Beethoven's Ninth Symphony Premiere | May 7, 1824 |
| International Migratory Bird Day | May 8, 2010 |
| National Windmill Day (Holland) | May 9, 2010 |
| Mother's Day Poems | May 9, 2010 |
| Mother's Day | May 9, 2010 |
| Mother's Day (postcard) | May 9, 2010 |
| Central and Union Pacific Railroads Meet | May 10, 1869 |
| Children's Book Week | May 10-16, 2010 |
| Richard Feynman's Birthday | May 11, 1918 |
| Edward Lear's Birthday (Celebrated as Limerick Day) | May 12, 1812 |
| Edward Lear's Limericks | May 12, 1812 |
| Start of California Gold Rush | May 12, 1848 |
| Jerusalem Day | May 12, 2010 |
| Jumping Frog Jubilee in Angels Camp, CA | May 13-16, 2010 |
| First permanent British colony Established in Jamestown, Virginia | May 14, 1607 |
| Lewis & Clark Leave St. Louis | May 14, 1804 |
| Major Lassen Peak Volcanic Eruption | May 14, 1915 |
| L. Frank Baum's Birthday | May 15, 1856 |
| Discovery of Largest Prime Number Known to Man | May 15, 2004 |
| Sue Debuts at Field Museum | May 17, 2000 |
| Karol Wojtyla Birthday (Pope John Paul II) | May 18, 1920 |
| Major Mount St. Helens Volcanic Eruption | May 18, 1980 |
Circus Day, Anniversary of First Ringling Brothers Performance | May 19, 1884 |
Premiere of Star Wars: Episode I | May 19, 1999 |
Charles Lindbergh Flies Historic Solo Nonstop Transatlantic Flight | May 20-21, 1927 |
| Amelia Earhart's Atlantic Crossing | May 20, 1932 |
| Weights and Measures Day | May 20, 2010 |
| Most Powerful Earthquake of the 20th Century Struck Chile | May 22, 1960 |
| World Turtle Day | May 23, 2010 |
| Hurricane Preparedness Week | May 24-29, 2010 |
| National Backyard Games Week | May 24-31, 2010 |
| Cellophane Tape Patented | May 27, 1930 |
| Golden Gate Bridge Opened | May 27, 1937 |
| Sojourner Truth Gives "Ain't I Woman?" Speech | May 28-29, 1851 |
| John F. Kennedy's Birthday | May 29, 1917 |
| President Woodrow Wilson Proclaims June 14 Flag Day | May 30, 1916 |
| First Daily American Newspaper Published | May 30, 1783 |
| National Spelling Bee Week | May 30-Jun 4, 2010 |
| Walt Whitman's Birthday | May 31, 1819 |
| Mark Felt Reveals Himself as Watergate's Deep Throat | May 31, 2005 |
| Memorial Day | May 31, 2010 |
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