Paris was nicknamed the “City of Light” (not City of Lights) originally because it was a vast centre of education and ideas during the Age of Enlightenment. In 1828, Paris began lighting the Champs-Elysées with gas lamps. It was the first city in Europe to do so, and so earned the nickname “La Ville–Lumière” or The City of Light.
Nevertheless if you call it "City of Light" or "City of Lights", I think both describe this amazing city really well!
Did you know that you are not supposed to share any images of the Eiffel Tower lit up at night? The lights that illuminate the attraction at night are technically an art work, so 'reproducing' requires the permission of the artist.
Curious to know more? Read this full article at: http://www.dailymail.co.uk/travel/travel_news/article-2831331/Tourists-warned-breaking-law-taking-photos-Eiffel-Tower-night-sharing-images-Facebook-ILLEGAL.html