Singapore is located only about 200 km north of the equator thus days and nights are about 12 hours long at any time of the year. Temperatures range from 25 to 34 degrees Celsius and almost never drop below 20 degrees Celsius. The lowest recorded temperature in Singapore was 19.4 degrees Celsius. Singapore is characterised by humid climate and frequent heavy showers and thunderstorms. There is no distinct monsoon season in Singapore although November to January tends to be the wettest and May to July the driest period. The difference in rainfall between the wettest and the driest period is minimal.
Based on the aforementioned, Singapore's climate doesn't represent a major decision factor on when to travel to Singapore. Whenever you travel to Singapore you will experience heat and high humidity which will make you tired easily. However, you can always escape from the heat, humidity and sudden showers into air-conditioned shopping malls, shops, restaurants and other businesses, where air-conditioning is normally set to the lowest possible temperature. If you intend to spend a longer period of time in an air-conditioned place (e.g. in the cinema) you might even need a sweater.
The decision on when to visit Singapore probably depends on air ticket prices and various events and festivals in Singapore. Shopping addicts should visit Singapore in May/June, during the Great Singapore Sale. If you wish to cheer up to the New Year in short sleeves then visit Singapore at the end of the year and marvel at beautiful Christmas decorations – tropical style. If you are into ethnic festivals, try to visit Singapore during Hari Raya Haji (normally in January or February), Chinese New Year (normally in January or February) or Indian festival Deepavali (in October or November).