Every spring and fall the White House’s gardens and grounds open to tourists. Tickets are free and are offered on a first-come, first-serve basis. The tickets are timed and are distributed by the National Park Service at the Ellipse Visitor Pavilion the day of the tour. I have had the opportunity to visit the gardens (see photos below) and I believe that is a MUST DO if you are in Washington, DC, the weekend of the gardens and grounds tour. I was unable to find the dates for the 2012 spring tours, but the tours tend to be in April so continue to check online for the 2012 schedule.
