Most wedding consultants advise that you book your musical talent soon after you set a date, at least 6-12 months in advance. A good rule of thumb is the larger the ensemble the farther ahead you'll need to book.
For example, if you plan to book a large ensemble like a choir or an orchestra, expect to book 12 to 18 months ahead.
Once you've chosen musicians start thinking specifically about what you'd like them to play. If you have special requests for your musicians allow enough time for them to learn and practice any new music. Again, the larger the ensemble the more time needed to put together a new piece. Throwing together a tune at the last minute is not advisable.
You want your musicians to be well rehearsed and give a polished performance.