To me December represents the nadir of garden activity. It's cold, the days are short, and frankly I'm too worried about surviving Christmas with the kids to think about plants. As far as gardening is concerned, January is a huge improvement over the preceding month.
Maybe you get them sooner, but the companies I buy seeds from send me their propaganda in January. I can't think of anything that instills more of a sense of optimism and confidence in one's ability to raise a garden than a seed catalogue. Thankfully my lousy soil and uncooperative weather conspire to ensure I am fully humble by August when I'm left with not much more than some basil, cherry tomatoes, and maybe some sunburned peppers.
But today I'm excited! August is a long way off.
This weekend I'll probably put in my order for seeds for 2014 if I don't have to put in too much time toward child supervision.
This is the time of year you need to start thinking about what you want to grow and where you'll get the seeds or transplants. I usually just use what ever tomatoes and peppers I can find transplants for locally, but when I want a specific cultivar, then I will need enough lead time to grow the transplants in order to have them ready in time for spring. The time to sow vegetable transplants is getting close. If your seed order is still a to-do item, you may miss your chance to grow your own transplants. Don't delay! Operators are standing by.
You probably need to sow about 6 to 8 weeks before transplants are set out in the garden. If you grow cole crops from seed instead of buying transplants that means you need to be sowing soon!
Be sure when you make your seed order that you get enough for your fall crops. It is hard to find seeds or transplants in late summer and the seed companies may be sold out of the kind you want.
Remember that seed companies' writers always describe their offerings as amazingly super-awesome, but clearly not all can be. I recommend experimenting a little because that's fun, but don't be afraid to grow tried-and-true cultivars that worked for you in the past. Likewise when you can't find your favorite, don't worry about it. They usually drop poor sellers that either perform poorly compared to their other cultivars, or are just too hard to produce. In other words, there's usually a good reason you can't find it.
You can always ask a Master Gardener or a friend what they grow, as well. In some cases it just doesn't matter at the small scale of a home garden what you pick. I've always been impressed with eloquent descriptions of items like radishes. Is there really a radish worthy of a J. Peterman treatment?
Most seed companies buy many of their seeds in bulk from suppliers and repackage them. If you see the same cultivar available elsewhere–and it's packed for the current year–don't be afraid to go with the cheaper option. Sometimes you're just paying for a fancy seed envelope, shiny catalogue, or great story. (Ask me about kiwanos some time.)
Finally, plant what you like to use. If you don't like turnips, they don't need to be in your garden. Plant more of what you like instead! I find myself planting radishes, eggplant and squash each year because that's what you grow in a garden, but I really don't like them. Every spring I think that this will finally be the year I like squash, but it has yet come to pass. As I'm fighting squash bugs in July I annually conclude that I would have been happier with that space planted in bell peppers.
Maybe this will be the year I only buy seeds for what I like and will grow, and do it early enough to plant on time. But don't bet on it.
I'm sure you'll do better than me.