I'm relatively new to the home built system world. I'll be looking to buy some components in the near future, but I missed the boxing day sales. generally speaking, when is the next good time to buy parts after boxing day.
If you're not buying the top of the line equipment, the best time would be after new technology is released. So in a few months when nVidia releases it's newest cards, it will be a good time to buy older tech, as the prices will (should) come down. The same will (should) be true when Intel releases the next CPUs. This may not happen if the new stuff isn't very good, in which case there will only be a temporary price drop while people figure that out.
Generally, I recommend that if you need new equipment now, you shouldn't play the waiting game UNLESS something is due out very soon or a sale has been announced. That said, if you don't need to have something in the next 6-12 months, I would wait around and pick up miscellaneous parts when they're available at a good price. Things like PSUs, cases, and optical drives will rarely change in price much. HDDs will change more slowly or have a lessened effect on performance. Basically, you should wait as long as possible before buying the CPU, motherboard and GPU.
There will be other sales that will occur over the next several weeks, but the deals will not likely be as good as the boxing day sales.
Your best strategy at this point is to figure out exactly what components you want to buy, and purchase them over a relatively short period of time (30 days) when you spot sales. For instance, you should be able to buy your case, power supply, and hard drives early because the chance for getting faulty components is small. Purchase you CPU, motherboard, RAM, and GPU last of all so that you can put the whole system together quickly within the return period of the supplier if you run into dead components.