A deposit (acconto in Italian) is an advance payment towards the total you will owe. It is usually paid when you sign the contract or accept the quote, to secure the date. The rest is paid as the balance, often a few weeks before the wedding or just after.
In Italy it should not be confused with a caparra, which has different legal effects if one party pulls out. Check what the payment is called and what the contract says about cancellation or a change of date. The guide on how to read a florist's quote explains deposit and balance; the one on the contract with your florist lists the clauses; the one on when to book says when it makes sense to pay it.
What to ask: how much the deposit is, when the balance is due, what happens to the deposit if you change the date.
