Paris museums: timed tickets, and the free days

The Louvre requires a timed-entry ticket even on free days, and the popular slots sell out days ahead, so book before you fly. Many national museums in Paris, the Musée d'Orsay among them, are free on the first Sunday of the month; the Louvre instead opens free on the first Saturday evening. Those days are also the most crowded, so a paid weekday morning can be the better experience.