I like to visit Paris when the weather is mild but the without the big crowds. Here is my run-down
January-February: The weather can be chilly with occasional snowflakes. However, small crowds, cheap airfares and hotel bargains make this the time to visit. Avoid visiting during large trade shows or events like Fashion Week.
March-April: If not visiting during Holy Week, the big crowds have not yet arrived and moderate prices rule the day at most hotels. But travelers will need to bring umbrellas. There seems to be a lot of school groups visiting at this time.
May-July: Hotels are expensive and airfares are sky-high. The weather can be a guessing-game with a significant amount of variability in precipitation and temperature. It will be your fault if choosing to visit during the Roland Garros French Open tennis tournament. If the Paris Air Show is in town, do a flyover and visit some other time.
August: Parisians abandon the city and go on holiday. This presents a good opportunity for travelers in terms of hotel bargains and the absence of big crowds, but airfares are still high. It’s the driest month of the year. Although some shops and restaurants are closed, most everything else is open as usual. Get a hotel with A/C and I think you will enjoy Paris in August. On my last trip to Paris, I visited in August.
September-October: My favorite time to visit Paris. It can be crowded with adult travelers but the weather is good, airfares have eased. Paris has a casual and relaxed feel to it.
November-December: The days are short and I’m not sure if a night-time Seine River cruise is the way to go, but the crowds are gone and airfares bottom out. The holiday displays at the department stores are a delight. Interspersed among dreary days will be a spectacular day or two of good weather.