I would strongly recommend that you do not book any wedding photographer until:-
Please have a look at the Q&A link, this lists many of the questions I'm often asked by couples and you may wish to ask your short listed photographers.
If your wedding is on a Saturday and between April and September, then it is also advisable to book early if you wish to secure the services of a 'good' photographer. Most bookings are taken 12 months in advance and a few couples that really like my work get into my diary 24 months before their wedding.
Most photographers give significant reductions for mid week (Mon to Fri) weddings and also for 'last minute bookings' (within 6 weeks of the wedding) just to fill in any blanks in their diary, but here you take the risk of not getting the photographer you want or any competent professional at all.
I'm always willing to give couples 'first refusal' while they get their plans in place, but to be absolutely fair for any dates on which I have multiple enquiries, the first booking form received with deposit will secure the day.
"Thank you so much for these photographs! They are beautiful. We have been enjoying showing them to friends and family. We had a wonderful day, and you were absolutely brilliant - many of our friends said so also.... HARRIET & MATTHEW