For tourists, there are two best seasons to visit Darjeeling town, one during the Spring which is short lived from March to April, and the other during Autumn (September to November).
These are the times when the temperature is pleasantly cool and the sky is usually clear. The Summer is short from May up to June. By June the Monsoon starts and usually lasts through August.
It can sometimes stretch even up to October. The winter is from December until February. Winter in Darjeeling can be very cold after December.
It's difficult to predict and rely on Darjeeling's weather. Although you may see a perfectly settled and beautiful weather at one instance, soon cloud or fog can rise from the deep humid valleys and put a white blanket all around.
Sometimes you can see flakes of cloud in the sky above you and at the same time some below you at the valley.