High season runs November to March and is dry, clear and about as reliable as Thailand gets. Mornings are pleasant, humidity is at its lowest, and the long lines at Erawan and Hanuman World hand you the views you came for without any haze in the way. What you pay for that is company, because this is when the parks are fullest and the popular pickup rounds sell out, so booking a day or two ahead stops being optional. Anyone travelling between Christmas and mid February should book earlier than feels necessary.
April and May are the hot season, which means the quietest courses and the hottest backs. At most parks ziplining is not strenuous, since you are hanging in a harness rather than working, but walking and climbing between platforms in 35 degree heat is another matter entirely, and a physical course like Jungle Xtrem in April becomes a serious undertaking. Take the 08:00 or 09:00 round and drink more water than seems reasonable and it is a perfectly good time to come.
Then there is green season, roughly May to October, which is our own favourite. The jungle is properly green instead of dusty, the parks are quiet, and the rain bears very little resemblance to what people picture when they hear the word monsoon. What Phuket actually gets is heavy bursts, usually later in the afternoon, so book a morning round and most days you will not see any.
The short version: Phuket ziplines run all year, the parks are at their prettiest in green season, and the time of day matters more than the month. Here is the longer answer.
Light rain does not, and the parks carry on running straight through it, which is no bad thing given that flying through warm rain in a jungle makes a better story than flying through sunshine.
Thunderstorms are a different matter, because steel cable at height and lightning are not a combination anybody plays with, so the parks pause when there is lightning in the area and start again once it has moved on. Most of those delays run 30 to 60 minutes rather than writing off the day, and in the rare case a park cancels outright you are moved to another slot or refunded, since nothing was prepaid.
Where rain genuinely changes things is on the ground rather than on the wire, because paths and wooden platforms turn slippery, which is why shoes with decent grip matter far more in green season than they do in January.
Morning, if the choice is yours, and mostly because it is cooler, which matters more than any of the other arguments put together. The light through the canopy is also better for photographs, the groups tend to be smaller, and in green season the afternoon is when the rain turns up.
There is a real case for afternoons though, if a smaller one. The late light is lovely, the last session is usually the emptiest of the day, and if you are the sort of traveller whose morning starts slowly then a 15:00 slot beats an 08:00 pickup you have come to resent.
In high season, one to three days. The morning rounds go first and the parks run a fixed number of vans.
In low season you can often book the same day, though we would still do it the night before to lock the pickup. Booking early costs nothing here: you reserve at the listed price and pay at the park on the day, and cancellation is free, so there is no reason to leave it late.
Phuket's parks have a last admission time, not just an opening time. Hanuman World stops admitting at 15:00, Flying Hanuman at 16:00, Jungle Xtrem at 16:30 even though the gate says 18:00. A late-morning plan that drifts can turn into a park that will not let you start the course. If you want an afternoon slot, book the session rather than assuming you can turn up.