I like to visit Paris when the weather is mild but without the big crowds.  Here’s the 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.  Stay away during large trade shows or events like Fashion Week.

March-April:  If not visiting during Holy Week, the hordes of tourists have not yet arrived and moderate prices rule the day at most hotels.   However, 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. The days are long and sunny.  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 August in Paris.

September-October:   My favorite time to visit.  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.