Mayor Marty Walsh, Gov. Charlie Baker and other officials unveiled commemorative orange banners bearing a white heart and the word "Boston" on them on Wednesday morning at the site of the blasts on Boylston Street.
A moment of silence follows at 2:49 p.m., when the first of two bombs exploded near the finish line on April 15. Church bells will then ring throughout the city.
Walsh has declared April 15 "One Boston Day," a new tradition in which Bostonians are encouraged to show kindness and generosity.
Three people were killed and more than 260 others were wounded in the attacks. The surviving bomber, Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv), has been convicted of 30 counts during his federal trial and could face the death penalty.