Malaysia is tropical, so it’s warm and humid year-round (temperature usually around 30°C/86°F in the day). There isn’t a summer/winter distinction, but there are Monsoon seasons that differ by region. On the East Coast (places like Tioman Island, Perhentian Islands, and Kota Bharu), the wet season is roughly November to February , during these months some island resorts even close due to heavy rain.

That period tends to be drier on the West Coast (Kuala Lumpur, Penang, Langkawi), which makes November–February a great time for west coast destinations​ The west coast gets more rain roughly April to October, while the east coast is sunny then​. That said, it can rain at any time in the tropics, usually as brief downpours. I visited KL around October last year and out of 5 days had few downpours two or max three times. 

Generally, many travelers find December to February great for most of Malaysia (west coast is dry and it’s peak season with lots of events, though do expect crowds). If you’re focusing on the east coast islands, then June to August is ideal (dry and sunny on that side). But even in rainy season, you might get sunny mornings with afternoon storms. So, you can really visit Malaysia year round.