In the Christian culture, a black arm band is worn in mourning or in protest.

Pledge to wear a black arm band on Inauguration Day, January 20,2009 from noon to three p.m. EST (make the adjustments from Atlantic to Pacific zones).

Also in the Christian tradition, on Good Friday, we especially observe three hours....noon to three. In this sense, the inauguration will have been completed, and the parade already passed.