Delos is famous for both mythological and actual historic reasons. It was supposedly the birthplace of Apollo and his sister Artemis, and was thus revered as a sacred site from earliest times. The mighty statue of Apollo was considered to be the most sacred site in Greece (only the base survives today). The island was thus a destination for countless pilgrims, and not only hosted a famous oracle and the Delian Festival, but also the main slave market of the Mediterranean world. During the time of the Delian league and the Athenian hegemony on the seas, it was the location of the league's huge treasury. The island also flourished during the early Roman era, when it was a multinational trading port with about 30,000 inhabitants, while today it is uninhabited except for wardens and archaeologists. Today it is the largest archaeological site in Greece and makes a great excursion from Mykonos, only a 30-minute boat ride away. There are several crossings from the Old Port in Mykonos every day. Don't miss the museum - it's included in the ticket price and holds the original lion statues and the original tiger mosaic. I bought my boat ticket a few hours before at the ticket booth, but it was mid-September, and maybe not that crowded anymore. In summer, advance booking may be a good idea. Be aware, though, that there is no shade on the island (except in the museum), and going for the midday tour is probably not a good idea. Adding a guide to your booking is also recommended, otherwise it would be hard to make sense of all the archaeological remains.