The "Sunday truce" is an agreement reached between the narcotics trafficking organizations of West Baltimore.
The truce, unwritten and referred in to Season Three, prohibited the use of violence against rival gang members or other enemies on Sunday.
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 (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 the shooting, ruined when it's run over by the taxi she and Omar were getting into. ("Slapstick")
Avon Barksdale, respectful of the truce, admonishes both Gerard and Sapper and orders them to buy a new hat for Omar's grandmother.