N.B. I'm reporting information found on the web in French, which looks broadly consistent and up-to-date. I've never visited these caves.
The Lascaux cave, or more precisely the partial replica Lascaux II that's open to the public, is open everyday during the summer period, and closed Mondays and for lunchtime outside the summer period. Check the web page for up-to-date information (grotte = cave, billetterie = ticket sales). In 2014:
- From Apr 01 to Jul 04, the caves and ticket sales are open 9:30–18.
- From Jul 05 to Aug 31, the caves and ticket sales are open 9–19.
- From Sep 01 to Sep 14, the ticket sales are open 9:30–18.
- From Sep 01 to Nov 02, the caves are open 9:30–12 and 14–18 everyday.
- From Nov 03, the caves and ticket sales are open 10–12:30 and 14–17:30, and closed on Mondays (except during some school vacations).
Note that during the summer period (in 2014: Apr 12 to Sep 14), tickets must be bought in the Montignac town center, next to the visitors' center. During the winter period, ticket sales take place at the cave entrance and have the same opening times (except that sales close shortly before the start of the last visit).
During the high summer period only (in 2014: Jul 10 to Aug 20), you can buy tickets up to 4 days in advance in Montignac. There is no way to book tickets online. At that time, it is recommended to come later in the week (preferably on a Saturday) and to buy tickets early in the morning or in advance. If you arrive in the afternoon on a busy day, you might queue 2 hours for a ticket and wait a further 3 hours to enter the cave!
Visits are guided tours only, lasting 40 minutes. There are tours in English all year round, and other languages during high season.
You can also visit Le Thot, which is about 10km away and hosts a few reproductions of paintings that are not presented in Lascaux II.