World famous for the shafts of light that turn this slot canyon into a wonderland of swirling, striated red and orange sandstone, Upper Antelope Canyon is not only a magical place for tourists but also a spiritual place situated on land owned by the Navajo people - which is why you can only visit on a guided tour. The best tours for light beams are generally between 11:00 am and 1:00 pm, but, as there's no limit to the number of visitors, the canyon will be busy (and the keener photographers can even get a bit aggressive).
If you go:
There are a number of operators - all based in nearby Page - offering tours; Antelope Slot Canyon Tours - operated by Chief Tsosie - provide a professional, helpful and friendly service.
Links:

Upper Antelope Canyon
Page, Arizona 86040 USA
ac@navajonationparks.org
Mailing address:
Antelope Canyon Park Office
P.O. Box 4803
Page, AZ 86040 USA
Phone Numbers:
Antelope Canyon Park Office
+1 (928) 698-2808
+1 (928) 698-2808