Singapore sits one degree north of the equator. The temperature barely varies — it is hot and humid every day of the year, ranging from about 25°C at night to 32°C in the afternoon. There is no cool season to wait for.
What does vary is the rain. Singapore has two monsoon seasons. The Northeast Monsoon runs from December to March and brings the heaviest rainfall, particularly in December and January. The Southwest Monsoon runs from June to September but is generally less intense. In between — February to April, and again in July to August — rainfall is lightest and the weather is as agreeable as it gets.
In practice, Singapore's rain usually arrives as sharp afternoon thunderstorms that last an hour and then clear. It rarely ruins a day of sightseeing. What you want to avoid is extended grey drizzle, which is most common in December and January.
February and March are probably the best months. Rain is lighter, and Chinese New Year brings extraordinary atmosphere to Chinatown — lanterns, red decorations, the Chingay Parade on Marina Bay, and fifteen days of celebrations. Book accommodation well in advance if your visit coincides with Chinese New Year.
June to August is school holiday season for Singapore and much of the region. Crowds increase at major attractions and prices for flights and hotels go up. National Day on 9 August brings a large parade and fireworks; the rehearsals in the days before are free to watch and nearly as spectacular.
October to November brings Deepavali to Little India — the streets are strung with lights and the bazaars overflow with food and goods. A quieter and often cheaper time to visit than the big Chinese festival periods.
December is festive and crowded. Orchard Road is strung with elaborate Christmas lights (a cheerful incongruity in 30-degree heat), and year-end school holidays keep attractions busy.
There is no truly bad time to visit. If you have flexibility, aim for February–April or July–August. Whenever you go, carry a small umbrella.