A steady supply of business travelers keep occupancy rates high year-round in Singapore. However, some hotels report that business travel gets sluggish during the months of July and August, when they target the leisure market more aggressively. This is probably your best time to negotiate a favorable rate. Peak season for travel falls between December and June, with "super-peak" beginning in mid-December and lasting through the Chinese Lunar New Year, which falls in January or February, depending on the moon's cycle. During this season, Asian travel routes are booked solid and hotels are maxed out. Favorable deals are rare because much of Asia takes annual leave at this time.
Because Singapore is 137km (85 miles) north of the Equator, you can pretty much guarantee that it's hot. In terms of seasonal variations, you've got some months that are not as warm as others, but for the most part, they're all still hot.
What does vary greatly is rainfall. Singapore lies between two monsoon winds. The Northeast Monsoon arrives the beginning of November and stays until mid-March, when temperatures are slightly cooler, relatively speaking, than other times of the year. The heaviest rainfall occurs between November and January, with daily showers that sometimes last for long periods of time; at other times, it comes down in short, heavy gusts and goes quickly away. Wind speeds are rarely anything more than light. The Southwest Monsoon falls between June and September. Temperatures are much higher and, interestingly, it's during this time of year that Singapore gets the least rain (with the very least reported in July).
By and large, year-round temperatures remain uniform, with a daily average of 81°F (27°C), afternoon temperatures reaching as high as 87°F (31°C), and an average sunrise temperature as low as 75°F (24°C). Relative humidity often exceeds 90% at night and in the early morning. Even on a "dry" afternoon, don't expect it to drop much below 60%. (The daily average is 84% relative humidity.)
There are 11 official public holidays (Note: The following dates, with regard to religious holidays, are estimates, as each date is subject to a different religious calendar. Dates provided here are for rough planning only. Please check with the STB prior to your trip to verify holidays that fall during your stay.): New Year's Day (Jan 1), Chinese New Year or Lunar New Year (Jan 23-24, 2012), Good Friday (Apr 6, 2012), Labour Day (May 1), Vesak Day (May 7, 2012), National Day (Aug 9), Hari Raya Puasa (also called Eid al-Fitr, Aug 30, 2011; Aug 20, 2012, observed), Deepavali (Oct 26, 2011; Nov 13, 2012), Hari Raya Haji (also called Eid al-Adha, Nov 6, 2011; Oct 26, 2012), and Christmas Day (Dec 25). On these days, expect government offices, banks, and some shops to be closed.