If you're interested in knowing what's in-season this month, here is a great link from the Ferry Plaza Farmer's Market/Cuesa to show seasonal produce, flowers & plants, and meats & cheeses! The charts are extensive but a short list derived from another source is listed below.
Eating seasonally means enjoying the ripest and freshest food at the height of its natural harvest time. If you shop at a local farmer's market or buy local items from grocery stores, it means supporting local farmers who transport their goods the shortest distance to your plate. It also means experimenting with delicious foods you might not otherwise consider adding to a meal!
http://www.cuesa.org/seasonality/charts/vegetable.php
December: avocados, navel oranges, almonds, walnuts, apples, cranberries, dates, grapefruit, persimmons, tangelos, tangerines, broccoli, brussels sprouts, cabbage, carrots, cauliflower, celery, mushrooms, parsley, rutabaga, spinach, sweet potatoes, turnips, winter squash, yams. (This list applies to CA definitely, and may apply elsewhere as well)
Eat up!