The Grand Union Flag was raised by George Washington on January 2, 1776 at Prospect Hill near his Cambridge headquarters during the Siege of Boston. The Continental Army was just reorganized on January 1 under the command of General Washington and this was the first flag the new army saw flying above them. Prospect Hill, now a part of Somerville, Massachusetts was a high spot in the area, so the flag could be seen for miles around.
Once a year, on the anniversary of the occasion, the citizens of Boston commemorate the raising of the Grand Union Flag with a ceremony on Prospect Hill. This video shows clips of the 232nd raising of the Grand Union Flag at Prospect Hill.