Booking a DJ is a task that not many people do regularly, even less regular for booking a DJ for your wedding.
So, when is the best time to book your DJ? Like any supplier, the best time to book a wedding DJ is as soon as possible. The later you leave it the less chance there is of the DJ you want being available.
Once you've chosen your date and venue I'd recommend looking to book the key suppliers that you know you want. The last thing you want to be doing is struggling in the last few weeks trying to book a DJ (or any other supplier in fairness).
Although it may be tempting to leave booking a DJ for a while to save paying booking fees, perhaps it's worth thinking about how much it'll cost if it goes wrong; priceless will no doubt be an answer.
Many people haven't booked a DJ before so I'd recommend having a look around and getting an idea about what DJs offer and what you value the most. Perhaps even pop along to a wedding fayre and have a face to face conversation.
Equally, I'd suggest not rushing to book any supplier just because of a "good deal" being offered. It's your one special day and you want to get everything right so you'll need to take your time and make sure you make the best choice for you.