Reading the wet season
Singapore is hot and humid year-round — there's no real "cool season." The main variable is rain: November to January brings the heaviest monsoon rain, while the rest of the year sees the more typical pattern of a short, intense afternoon downpour followed by clear skies.
School holidays & crowds
Local and regional school holidays (notably around June and December) push up hotel prices and attraction queues, particularly on Sentosa and at the Zoo. Shoulder months outside these windows are calmer and often cheaper.
Festival timing
Chinese New Year (Jan/Feb) brings Chinatown alive but also closes some smaller businesses; the Formula 1 Grand Prix (September) fills hotels and closes roads downtown; Deepavali and Hari Raya bring their own neighbourhood light-ups in Little India and Kampong Glam.
The short answer
February to April tends to be the sweet spot — past the New Year crowds, before the heaviest mid-year humidity peaks, and outside the F1 price spike.