The average temperature in Singapore varies between 20 and 30ºC, and rain rarely falls below 100 mm per month.
The rainy season
Although it's difficult to tell the difference between the seasons, and rain is common, it is true that between November and January it does tend to rain more than in other months.
Best time to visit Singapore
Any time of year is good to visit Singapore. Instead of going based on the climate, it's a better idea visiting based on the local festivities, or on the price of the plane ticket.