It is just before dawn. Capulet, Lady Capulet, and Paris have stayed up late, discussing Juliet and the tragedy of Tybalt's death. Paris turns to go, but Capulet suddenly calls him back. Capulet says he wants Paris and Juliet to marry, and thinks her heart "will be ruled in all respects by me" (3.4.14). He suggests the marriage be held on Wednesday. Then, realizing its Monday, he says Wednesday's too soon and moves the Wedding back to Thursday. Paris is overjoyed. Capulet instructs Lady Capulet to tell Juliet about her imminent wedding.
Juliet's apparent grief at Tybalt's death (which is really grief at Romeo's banishment) moves Capulet to decide to marry her to Paris. Capulet is trying to make Juliet happy, but this is a decision he's making for her. As her father, he has that right, and expects total obedience. Interesting that all this is happening at dawn—the moment when night shifts to day.