You can visit Cameron Highlands anytime of the year! However some periods are better than others. Generally year end tend to be wet due to the North-East monsoon blowing in. However tropical Malaysia weather is notoriously difficult to predict. Off peak season is the best when accommodation prices are cheaper, and you don't have to rub shoulder to shoulder with the crowds. But then again, it depends on your available schedule, not withstanding you may have to coincide with your school going children holiday term break and match with weekend holidays. But such periods are best avoided if you can.
Why would I suggest off-peak season to go Cameron Highlands? Simple! That's when everybody is not cramming in their vacation holidays. You get to enjoy Cameron Highlands all by yourself. But planning for a trip during an off-peak season is not for everyone due to commitment to work, school going children, and who knows what. Makes it difficult to plan for a trip.
For those who can tear themselves away during off-peak period you are well rewarded.
And the drawback? Not much except the Brinchang night market is not available - but who cares if you just going there to relax!
When going there during the super peak period see the getting there to Cameron Highlands page to find out which route you should use to visit Cameron Highlands.