Washington DC is most beautiful when the cherry blossom trees are in bloom. The annual cherry blossom festival in 2009 is from March 28 to April 11 and is filled with family-friendly events that honor the cherry trees. April 1-4 are the dates this year that the trees reach their peak blossoming, filling the Tidal Basin area with flowers and tourists.
A hugely attended event is the 10k run on April 5th that offers participants incredible views of the flowering trees. There is also a 5k walk and a 1k walk for kids. Due to the popularity of this annual event, it is necessary to register early to run. If you missed registration this year, you could volunteer at the event and then are automatically entered into the 2010 run. Over 500 volunteers are needed to keep everything "running" smoothly.
The cherry blossom trees planted in 1912 were a gift from Japan to symbolize the friendship between Japan and the United States. Thousands plan visits to DC every year to see the pink and white blossoms, hoping for good weather and that the blooms will open on schedule.