Hi, Varanasi is my hometown so I think I can help you with this. It's generally quite hot in the summer months, that is April to June so you would want to avoid that. Also, the monsoon sets on between July-August so apart from these months, you can pretty much visit the place anytime.
The best time to visit Varanasi is Winter (November - February) since the temperature is low and there is a calm breeze throughout the day. Summers in Varanasi experience high and dry temperatures. It is best to avoid the summers since they are hot dry and sunny making outdoor sightseeing difficult.
Best Time is now!