Singapore has a hot tropical climate and has little variation in temperature throughout the year. From that point of view you could visit at any time, but it is worth remembering that the rainy season begins in November and lasts till January. At this time it will probably rain every day; however, there is a lot of rainfall at other times too so the difference isn't that great.
Another difference is that between July and October there is often a haze; this is caused largely by bush fires from Singapore's neighbouring country, Indonesia. There are daily forecasts issued about this. Some tourists prefer to avoid this period of the year.
On average, daytime temperatures in Singapore are around 31 degrees centigrade and at night it's about 24 degrees, with little change throughout the year. With the high humidity it can get uncomfortable in the middle of the day, which is one reason they invest in efficient air conditioning in most buildings.
You can learn more about visiting Singapore here.