That is entirely up to you, however the purpose of the rehearsal is to ensure that everything runs smoothly at the wedding ceremony. You will certainly feel more confident that the ceremony will be flawless if everyone in your bridal party is familiar with their role.
The rehearsal can be done the day before the wedding or a couple of days before, depending on what is most convenient for you and the rest of your bridal party. It is often followed up by a rehearsal dinner, but keep in mind, that is also optional. It certainly makes for a good time for everyone to get to know each other before the big day, especially if your families are not already well acquainted.