From the L.A. Times:

January is absolutely peak season for oranges from the San Joaquin Valley, the state's leading growing area. Citrus from this district, where each variety has a relatively defined season, takes a little more care to obtain at best, compared to fruit from Southern California, where citrus can hang on the tree much longer in good condition. Some of the best choices at this time are Page oranges, Washington navel oranges and Moro blood oranges.

Page is not a purebred orange at all, or even a mandarin, as it's sometimes called, but a hybrid of Minneola tangelo with a clementine, made by the USDA in 1942.

Market Watch: The peak of flavor for San Joaquin Valley oranges