Conclusion: the main free windows are the first Sunday of each month from October to March, plus European Heritage Days in September. ① Free days cover the permanent collections only — temporary exhibitions still require tickets. ② Under-18s and EU residents under 26 enter free all year. ③ The permanent collections of city-run museums are free year-round. ④ Free days are the busiest: expect 45–90-minute queues at the Louvre and Musée d'Orsay, so book a free timed slot online (usually released about a month ahead). ⑤ In peak season (July–August) queues can exceed an hour, making a paid timed ticket the better trade. Tip: reserve as soon as slots open and arrive before opening.