"Sunday truce" is an agreement reached between the drug dealing crime families of The Wire HBO series. The truce, unwritten and referred in to Season Three, prohibited the use of violence against rival gang members or other enemies on Sunday.
During the Season Three episode Slapstick, Barksdale soldiers Gerard and Sapper spot Omar Little, a longtime enemy of the Barksdales, taking his grandmother to Sunday mass. Knowing that the reward for Omar's death or capture is high, the two phone Slim Charles to get permission to kill Omar. Slim is unavailable so they phone Stringer Bell (the two actually speak to Stringer's assistant, Shamrock) who gives them the go-ahead. The two fail to kill Omar, who escapes with his grandmother, however, her favorite hat is lost in them shooting, ruined when it's run over by the taxi she and Omar were getting into.
Avon Barksdale, respectful of the truce, admonishes both Gerard and Sapper and orders them to buy a new hat for Omar's grandmother.