Roman is getting married to Mallorie, and he wants you at the wedding in a nice suit.
If you don't have one already, head to Purseus and buy a nice suit and some nice shoes, then make your way to the wedding.
Your Choices Will Come Back To Haunt You
Depending on whether you chose to accept the deal, or go for revenge, the mission will change.
CHOICE 1: Accept The Deal
If you accepted the deal, Kate won't show to the wedding because she's pissed off that you went for money instead of what you believed in. Once you enter the mission marker outside the chuch, a cutscene will begin and the ceremony goes ahead as planned. Mallorie and Roman wed, and as they move outside to meet with a few familiar faces (Brucie looks rather smart in a suit), a hitman approaches Niko and attempts to murder him as a "present from Dimitri". Niko attempts to disarm him but only manages to point the bullet straight at Roman who is killed. After putting a bullet of his own into the assassin's head, Niko takes out his anger on the dead gunman and returns home.
CHOICE 2: Take Revenge
If you took revenge, you'll need to pick up Kate before the wedding, so head to her Ma's house and collect her, then drive to the wedding and enter the mission marker. After the cutscene, Kate will be hit by a bullet shot from Pegorino and she will die. Niko blames himself for letting her die and returns home, distraught.
Time To End This
After the cutscene, Niko will get out of bed at home. Head outside. After a minute or two, Little Jacob will text you, telling you to get some hardcore weapons because you're going after those responsible.