Make sure you get in early before the date gets snapped up
No matter which day you get married, there will be other weddings going on in the area. That means that you are competing with all those weddings to get the entertainment you want.
There is no hard and fast rule about when to book your entertainment, only what works for you. In my experience though, most of my bookings come in 3 to 6 months in advance of the day. That doesn't mean you can't enquire with more or less time than that, that's just how most people do it.
Having said that, I have bookings going into 2019 and will take bookings for 2020 and 2021 if you really wanted!
Like I said, there is no set rule, I've taken bookings with days to go. I once got a booking with hours to go until I started. That's a story for a different time though!
Of course the further in advance you book, the more chance you have of securing the entertainment you want. The last thing you want is to be stuck with entertainment you don't really want at your wedding.
I know I probably haven't answered your question but in reality I would say as soon as you've found the right entertainment, book them up. You don't want to leave it and find they're already booked.
If an entertainer doesn't open their books that far in advance, I would be weary as to why but, get them to call you as soon as that date becomes available so you get first refusal. If they don't do that then they are probably not the right entertainer for you. You are super prepared and they aren't prepared to work with that.
I take bookings as far in advance as you want so you never have to worry about that!
All the best,