Matrimony is just a fancy way of saying "marriage." When a couple ties the knot, they are engaging in matrimony.
You can describe the actual wedding celebration as matrimony, and also the state of being married, although it's a formal word most often used in documents and in the words of the ceremony. The Latin word for matrimony is matrimonium, which comes from combining mater, "mother," with the suffix monium, "action or condition." In the old days, matrimony was basically the same thing as making a woman into a wife and mother.