The Festival of Lights or Fête des Lumières takes place every year around the 8th December as a celebration of gratitude towards St. Mary. I’ve heard several variations of the story concerning the event, but here is one short version of it:
Lyon’s citizens started the tradition of giving offerings and lighting candles in the Basilique Notre-Dame de Fourvière in 1643 for being spared during the plague. In 1852, it was decided to erect a statue of Virgin Mary next to the Basilique, and preparations took place for a big celebration on September 8th. Unfortunately, weather conditions prevented the event from happening, and it was postponed until the next religious celebration on December 8th. During the day, a storm struck Lyon and the inauguration could not take place.
Later at night, though, the storm passed away (like a godly miracle), so in order to (finally) put up the statue, the citizens of Lyon gathered with candles and lights, thus starting the tradition of putting up candles on the windowsill on December 8th.
Today, the festival attracts millions of visitors, encouraging artists and performers alike to showcase all sorts of light-related installations.
PS: A week later, there was a mini Fête de Lumières in Montpellier, there were light shows on three buildings in the old town, which was kind of cool, but in comparison to Lyon (there is no comparison, really), it was just trop mignon. As Montpellier does not really have enough space in front of its main buildings, there were busses and trams passing in front of the spectacle and people standing in the middle of the (not blocked) road.
Avoiding the same weekend may defer the crowds and attract more visitors, but defeats the purpose of the celebrations in my humble opinion (as mentioned above, there is a reason for it being on that specific date). Nevertheless, a welcome excuse to meet up with some friends and enjoy the crowds.