That moment when the music swells as you walk down the aisle sets the entire tone for the ceremony. Since the best groups book up fast, you should lock in your quartet at least six months out if you aren't doing a Saturday date.
A quartet usually runs about $800, which can be a big hit to your bank account all at once. Planning ahead lets you manage that expense without draining your savings before the big day.
The 18-month window is a deadline, not free money. If you have a remaining balance after the 18 months are up, you will start paying interest on whatever is left, which will quickly cost you more than the original price of the music.