If you mean how are in advance you should book. PP is correct, as soon as possible. Rates typically rise the closer you get to sailing. If rates have already starting rising on a sailing you are thinking about, there is a slim chance that they may offer a *GT rate within 75-90 days of your sailing but they aren't always available and you have to be flexible, including getting to the port to take advantage of them.
If you meant when is the best time to travel to get a good deal, then typically when schools are in session. Sept - Nov (also hurricane season so rates can be a little lower but you have to "go with the flow" if the weather doesn't cooperate.) January, after the holidays, is generally a less expensive time. There are weeks in Feb - Apr that can be cheaper but you have to avoid holidays (President's Day and Easter) and spring break times. So May can also be a less expensive time.