Anywhere between mid September-March.
bt make sure you plan your trip well in advance cos during diwali & makar Sankranti it's too crowdy.
people from all over India & evn foreigners visit during dis time for the holy dip in Ganges
Winter would be the best time to visit Kashi. Most off the boat rides will be closed during rainy season as the flow will be dangerous. Summer will be too hot. Try to book hostels or ashrams which are cheap compared to hotels. Try local food and pan. If you wanted to purchase saree bargain bargain bargain as you won't be able to figure it out which is original silk which is half silk. If you are visiting first time hire local guide for 300-500 bucks trust me he will be your financial advisor for temple visits, Bath etc. Thank me later.