I plan to travel to South-Africa at the end of the year (last week of December, first Week of January). I plan to spend more or less two weeks there. My understanding is that this period is really high season. Hence I wonder how difficult it will be to find an accommodation at that period? Are there places that are advised or disadvised?
Like anywhere in the world, being somewhere at high season may always put a strain on accommodation.
Most important is the game parks - if you're going to Kruger or similar, BOOK IT NOW. As soon as possible. With other cities or towns you can almost always find something, and depending what type of traveller you are, could even rough it in a vehicle or camp. However, you absolutely cannot stay outside of the special camps in Kruger and most game parks, so you'll need to book that now. I recommend Pretoriuskop rest camp in Kruger - it's easy to get to, and has a lot of drives with plenty of water holes nearby, very well located and reasonably priced.
Otherwise it's just like anywhere else - book ahead of schedule. I did a big trip in 2005, among other times I've been there, and we booked guest houses 1-2 weeks ahead - in March/April, and had little problem finding places.
Oh, also the mountains - any places in the Drakensberg, you'll want to book ahead of schedule too, as they have limited spaces. I recommend any of Giant's Castle, Dragon's Peak or White Mountain - all have beautiful walks and amazing scenery.