A quarter-million Girl Scouts are expected to descend on the D.C. area this weekend to take part in the organization’s 100th-year anniversary. The Rock the Mall event will feature musical performers and a 60-song sing-along at the National Mall on Saturday. Though they have held similar events on past anniversaries, organizers said this year’s will be one of the largest to date. Participants are expected to arrive from every state and some from other countries. Mount Vernon will welcome many of the scouts the following day, planning special programs at George Washington’s home.