Singapore has a hot and humid climate throughout the year. Basically the Island doesn’t have a fix set of season. Being on the Equator the weather is pretty stable year round with the average low of 20 degree Celsius to average high of 30 degree Celsius. There are two main monsoon seasons in Singapore: Northeast Monsoon Season (December-March) and the Southwest Monsoon Season (June-September). The other significant season is the inter-monsoon period (April-May and October-November).
So, the best time to visit Singapore is in the months of June to July and November to December.