Most District and federal offices will close Tuesday for a National Day of Mourning in honor of President Gerald Ford, adding a fourth day to the holiday weekend for thousands of people.

All federal agencies will be closed, with a few exceptions. Personnel within state, defense, justice and homeland security may have to work, at their directors’ discretion. Fort Belvoir, in Fairfax County, will be closed, as will the U.S. Citizenship and Immigration Services.