From the end of spring to the beginning of summer (April to June) and autumn is the best seasons to visit Paris.
Spring（Average Temperature 10.5℃-19℃）
This is a flower blooming season which allows you to enjoy a pleasant trip. People drink coffee or beers in the terrace because it's warm and sunny after a long winter. But please still remember to bring a light coat in case of cold wind.
Summer（Average Temperature 15.5°-24.4°）
People around the world flood into Paris for a beautiful summer. The weather is generally lovely but sometimes can reach 32 °C. It doesn't sound so terrible, but when it comes to no air conditioner, inevitably there gonna be some sweaty nights. Seine River is a good place to get tanned because every year there will be an artificial beach called Paris-Plages( Bikini time!). You can go there and enjoy a cold drink and sunshine.
Don't miss the firework on every 14th July for France National Day.
Autumn（Average Temperature 9.2℃-15.8℃）
Autumn has mild days and fresh nights but is changing and unstable. Don't be too surprised if there is some warm or cool weather, because it frequently occurs. So, bring shorts and some nice warm outfits as well.
Winter（Average Temperature 2.5°-6.9°）
In winter, sunshine is scarce; days are cold but generally above freezing with temperatures around 7 °C (45 °F). Light night frosts are however quite common, but the temperature will dip below −5 °C (23 °F) for only a few days a year. Snow falls every year but rarely stays on the ground. The city sometimes sees light snow or flurries with or without accumulation.
Information from https://en.wikipedia.org/wiki/Paris#Climate