Good question. I'm sure that it one that even experienced cruisers ask. There is so much emphasis placed by the companies to buy now or you could miss it and that is true of some, but o0ften a minority of cruises. Obviously peak summer time provides fewer oportunities for a good price for sailing out of the UK than other times. If you have a destination which is crucial to your plans I would say "Bite the bullet" and buy as soon as is practicable but I would like a £! for every time that I have said I wish I had held off a lot longer....just look at what we could have got. It is like stacks and shares, seldom do you buy them at their lowest price and sell them at their top price, so really the ball is back in your court.....sorry about all the mixed metaphors but I hope it helps.....Ida Dow.
There is no good time I'm afraid. But remember if you book with RCI/Celebrity then keep an eye on the price of your cabin [do this by pretending to do a new booking on their website]. If the price goes down then call your T/A and RCI/Celebrity will convert your booking to the new price. Not many people know this..... So far I have had £180 refunded since I booked in May - and I keep watching the price for further price drops.
Further to the other answers, our principle is that we never book any cruise until the offer is 50% or less, then don't wait. Once the cruise agents get given blocks of cabins there is very little time between going to press - my wife scoures the Saturday Mail & The Mail on Sunday - and you having to get second-best in your desired cabin/deck, or they have all gone. Another valuable point is the longer you leave it, while you may still get you preferred choice, the flights are full, & the agent will have to route you in a convoluted way. We know this to happen, personally.