When to go to Paris?

Every season has its advantages when it comes to taking photos in Paris, and each can convey a different mood to your pictures.

Spring

This is probably the nicest season to be in Paris. The days are getting longer, allowing for sessions of night photography before dinner. People go out more, taking advantage of the warmer temperatures. On the first good days of Spring, parisians seem to all go out on the streets, in cafes, on the quays, especially on weekends. There are festivals in May or late June.
In Spring, the weather can be changing. Some years can be very warm and dry. In 2011, it almost did not rain in April and May, but it was an exceptional year. It can be cold and rainy for weeks in April or May.

Summer

 An interesting season for many reasons. 
At the beginning of = Summer, Paris gradually gets quieter as parisians go on holiday. Mid to end of August is a time of reduced activity and empty streets. It is far easier to park in downtown Paris. 
The operation Paris plage and several festivals also take place in July and August. 
Days are still quite long in summer, which will give you more time to shoot and there might be thunderstorms, which bring interesting lighting.
The weather is generally fine with mostly sunny weather. There can be exceptions in specific years. The Summers of 2001, 2002 and lately 2011 have been wet and cold from the beginning of July to August.

Autumn

People come back from holidays on the first week of September, and things suddenly get very busy compared to August. But the most visited places also get less busy.
Trees get nice colors by mid/late October.
The late season can be quite good with warm and sunny weather in September and October, as in 2010.  In November, it generally gets quite cold, the trees have lost their leaves and days are really short.

Winter

Winter in Paris can be interesting. Despite the short days, you will not have to wait too long to get nice shadows, or, if you have a tripod to shoot night scenes.
Christmas decorations are everywhere, but do not miss the covered passages, the grands boulevards and of course, the Champs Elysées.
January is often a cold and dull month, but things usually get better with some sunny - though often cold - days in the beginning of February.
Snow is normally uncommon in Paris and does not last for long, but we did have some in January 2003 and in the winter of 2010/2011. In case of snow, be out early: it does not last. Try your luck with gardens and parks to get a pristine white cover.