| National Kite Month | April |
| National Frog Month | April |
| National Garden Month | April |
| Financial Literacy Month | April |
| California Earthquake Preparedness Month | April |
| National Humor Month | April |
| National Mathematics Education Month | April |
| National Poetry Month | April |
| Zoos: Zoo and Aquarium Month | April |
| Aquariums: Zoo and Aquarium Month | April |
| King George III Reacts to the Boston Tea Party by Closing the Boston Port | Apr 1, 1774 |
| April Fools' Day | Apr 1, 2009 |
| Firefighters Day | Apr 1, 2009 |
| Frederic Auguste Bartholdi's Birthday | Apr 2, 1834 |
| Ponce de León Discovers Florida | Apr 2, 1513 |
| Hans Christian Andersen's Birthday | Apr 2, 1805 |
| Death of Pope John Paul II | Apr 2, 2005 |
| Square Root Day | Apr 4, 2016 |
| National Week of the Ocean | Apr 5 - 11, 2009 |
| Booker T. Washington's Birthday | Apr 5, 1856 |
| National Week of the Ocean | Apr 5-11, 2009 |
| World Health Day | Apr 7, 2006 |
| Metric System Adopted in France | Apr 7, 1795 |
| National D.A.R.E. Day | Apr 8, 2009 |
| Civil War Ended | Apr 9, 1865 |
| Jumbo the Elephant Arrives in US | Apr 9, 1882 |
| Passover | Apr 9, 2009 |
| Safety Pin Patented | Apr 10, 1848 |
| Spain Cedes Gibraltar to Great Britain | Apr 11, 1713 |
| Jonas Salk's Polio Vaccine Declared Safe and Effective | Apr 12, 1955 |
| First Space Shuttle Launch | Apr 12, 1981 |
| Easter | Apr 12, 2009 |
Civil War Begins with Confederate Attack on Fort Sumter | Apr 12, 1861 |
| National Library Week | Apr 12-18, 2009 |
| Alfred Butts' (Inventor of Scrabble) Birthday | Apr 13, 1899 |
| Thomas Jefferson's Birthday | Apr 13, 1743 |
| Leonardo da Vinci's Birthday | Apr 15, 1452 |
| Titanic Disaster | Apr 15, 1912 |
| Pet Owner's Day | Apr 18, 2009 |
| Paul Revere's Midnight Ride | Apr 18, 1775 |
| San Francisco Earthquake | Apr 18, 1906 |
| National Park Week | Apr 18-26, 2009 |
Revolutionary War Begins with
Shot Heard 'Round the World | Apr 19, 1775 |
| National Volunteer Week | Apr 19-25, 2009 |
| National Playground Safety Week | Apr 20-26, 2009 |
| Holocaust Day | Apr 21, 2009 |
| John Muir's Birthday | Apr 21, 1838 |
| Lyrids Meteor Shower | Apr 21-22, 2009 |
| Earth Day | Apr 22, 2009 |
| Ecology Games for Earth Day | Apr 22, 2009 |
| Endangered Species for Earth Day | Apr 22, 2009 |
| Shakespeare's Birthday | Apr 23, 1564 |
| Arbor Day | Apr 24, 2009 |
| World Penguin Day | Apr 25, 2009 |
Hubble Space Telescope Launched by Space Shuttle Discovery | Apr 25, 1990 |
| Italy Liberation Day | Apr 25, 1945 |
Whydah Pirate Ship Sank Off the New England Coast | Apr 26, 1717 |
| Richter Scale Day | Apr 26, 2009 |
| Ulysses S. Grant's Birthday | Apr 27, 1822 |
| International Astronomy Week | Apr 27-May 3, 2009 |
| Vietnam Liberation Day | Apr 30, 1975 |
| U.S. Buys Louisiana Territory from France | Apr 30, 1803 |
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