Any time’s a good time to go to Singapore. There are cultural events and festivals all year round, from fashion to film to food, and because Singapore is home to so many ethnic communities, you can hardly step outside without bumping into a festival.
Practically on the equator, Singapore is constantly hot (the temperature never drops below 20°C), humid and gets fairly steady year-round rainfall. The wettest months are supposedly November to January, when it’s also a degree or two cooler, while the driest are supposedly May to July, but in reality there is little distinction between the seasons. Similarly, there is no high and low tourism season as such, though during local school holidays and major cultural festivals things become noticeably more crowded.