Best time to visit Paris is from May till June and from September till October, when the weather is mild, the city is not overcrowded, sightseeing gets more enjoyable, but hotel prices are relatively high.
High season in Paris is in July and August, so expect hot temperatures, a crowded city, long lines for sightseeing and expensive lodging.
For budget travel the months from November till February are ideal, but not during Christmas Holidays. You will get the best hotel deals, great sightseeing, but also a chilled weather.
March and April offer better hotel prices and less crowds, with some sunny days, an ideal period for sightseeing the city of light.