When they brought Joe's killer to justice.
Throughout Season 1, Jamie had been followed by the Blue Templar—a secret society of the NYPD that murdered his brother, Joe. Jamie and Danny went to Frank with everything they had heard and seen about the Blue Templar in the finale. After bringing Erin into the fold to get a wire tap, the band of Reagans were able to conclude—and prove—that the Blue Templar was behind many illegal activities, most importantly killing Joe. Once justice was done, the family had their usual Sunday dinner and all took a trip to visit Joe’s grave.