Ball at the Moulin de la Galette by Pierre Auguste Renoir, Musee d'Orsay.
Renoir was studying the effects of light at different times of the day at one of his favorite venues and as he enjoyed himself with his friends - good work if you can get it. One of the most famous of the Impressionists, he was working closely with Monet at this time. Check out Kahn Academy for some great videos on art and artists.