There are two peak seasons in Malaysia. One peak tourist season falls roughly from the beginning of December to the end of January, covering the major Northern Hemisphere winter holidays -- Christmas, New Year's Day, and Chinese New Year. Hari Raya Puasa, celebrating the end of Ramadan, shifts dates from year to year. If you plan to travel to Malaysia around August, I highly recommend calling Tourism Malaysia to find out exactly when this holiday will fall.
The second peak season falls in the months of June, July, and August, and can last into mid-September. During this period, hotels are booked with families from the Middle East, as this is school holiday season for many of the region's countries. After September it's quiet again until December. Both seasons experience approximately equal tourist traffic, but in summer months, that traffic may ebb and flow.
Singapore's school holidays occur from mid-May through to the end of June, and again during November and December, when families are likely to flock to Malaysia's seaside resorts, particularly the budget and midpriced properties. Malaysians in general are relaxed about children when on holidays. If you're looking for an intimate couples escape, try to avoid family resorts during this time. Malaysia's schoolchildren are cut loose for about 1 to 2 weeks in March, May, and August, with a longer break from mid-November through December.
Climate considerations will play a role in your plans. If you want to visit any of the east-coast resort areas, the low season is between November and March, when the monsoon tides make the water too choppy for watersports and beach activities. During this time, many island resorts may close. On the west coast, the rainy season is from April through May, and again from October through November.
The temperature is basically static year-round. Daily averages range from 73°F to 90°F (22°C-32°C). Temperatures in the hill resorts get a little cooler, averaging 67°F (21°C) during the day, 50°F (10°C) at night. In terms of rainfall, the wettest months are March and April, as well as October through December, with monthly average rainfall in the range of 10 inches. The driest months are June and July, with only about 5 inches rainfall each.
An additional consideration is haze caused by unfortunate slash-and-burn agricultural practices on the Indonesian island of Sumatra, with winds carrying smoke across the Straits of Malacca to the peninsula. By and large, haze will affect only blue skies and visibility, but it has reached dangerous levels, such as in 2005, when they soared to "state of emergency" levels with regard to human health. The burning period is roughly from August through October, but it's impossible to predict when and if haze will affect your holiday.
During Malaysia's official public holidays, expect government offices to be closed, as well as some shops and restaurants, depending on the ethnicity of the shop owner or restaurant owner. During Hari Raya Puasa and Chinese New Year, you can expect many shop and restaurant closings. However, look out for special sales and celebrations. Also count on public parks, shopping malls, and beaches to be more crowded during public holidays, as locals will be taking advantage of their time off.
Official public holidays fall as follows: New Year's Day (Jan 2, 2012, observed), Chinese New Year (Jan 23-24, 2012), Prophet Muhammad's Birthday (Feb 15), Labor Day (May 1), Wesak Day (May 7, 2012, observed), King's Birthday (Jun 4), Hari Raya Puasa (also called Eid al-Fitr; Aug 30-31 and Sept 1, 2011; Aug 20-22, 2012); National Day (Aug 31), Deepavali (Oct 26, 2011; Nov 13, 2012), Hari Raya Haji (also called Eid al-Adha; Nov 7-8, 2011; Oct 26-27, 2012), and Christmas (Dec 25). Note: Please confirm all 2012 dates listed above before you plan your trip; some holidays vary, as they are dependent on the phases of the moon. In addition, each state has a public holiday to celebrate the birthday of the state sultan.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.