Musicians and agencies probably all handle this a little differently. Here’s how I do it. You put your half downpayment in to reserve your musicians. Let’s face it, good musicians are busy, and to get the best ones, we want to book them as early as possible in the process.
If you decide, for whatever reason, to cancel your music ninety days before the contracted date, you will get your entire downpayment back. Sixty days, and now I really have to give the musicians something for holding your date, so you get half back, and I pay the musicians with the rest, because at this point they don’t know if they can get other work, and in fact they may already have turned down work to play for you!
Thirty days out, though, is just cutting it too close. I keep the entire downpayment and divide it up between the now unemployed musicians on that date. We all move on with our lives, and hopefully still live happily ever after.