The best way is to buy tickets in 2-3 weeks in advance because there are always many tourists. In addition, there is a chance that there will be no free spots left for the date that you need. Moreover, there is a commission if you buy tickets online, which is 4 euro per person. However, I recommend you overpay these 4 euro since it will save your time that you can spend in queues, especially if you are going to Rome during summer time’s heat. Tickets cannot be refunded. Don’t forget about the dress code when you plan your visit to the Vatican! Cover your shoulders and don’t wear short skirts, shorts or dresses. The Vatican Museum and Sistine Chapel are open Monday to Saturday, 9 am to 6 pm (final entry is at 4 pm).