Whatever year you're in, take a break! Make sure you catch up on some sleep, chill with your mates and let your brains relax so you can actually cope with the next term rather than be worn out before you even get back to uni. This is especially true for the first years, enjoy your holidays now. They won't be so relaxing this time next year so enjoy them as much as possible while you can.
Second Years
The second term in second year is one of the busiest of the degree so try to make sure you organise yourself over the holidays so you can tackle it head on. Three weeks off may not seem like a long time, but you can tackle a surprising amount in those three weeks if you plan your time. (I apologise if I sound like all those study skills lessons you had in sixth form, but it really is vital to be organised!) While you have until week 3 to hand in the fieldwork elements of portfolio one it is really not a good idea to leave it until term time to do it if at all possible. I can tell you from experience it certainly makes the beginning of that term a lot more stressful than it needs be!
Third Years
Its so important you do have a bit of a break. You won't feel like you have time but next term will be really busy so you need to start full of energy. Firstly make sure you have very clear notes on where you are with your dissertation and exactly what needs doing. However obvious or simplistic it is now write it down because you won't have much time to work on it over the spring term and you do not want to lose time at the end of term and over easter trying to work out what you are meant to be doing. Make sure you know your deadlines for your assessed seminars before you break up rather than leaving it until next term and then finding out you only have 3 weeks to prepare it all. If you have the time (I know you're really busy with special topics and the exam) talk to your course supervisor to make sure you can do the topic you want to do so you don't waste your time and effort planning something you then need to scrap.
It may seem like I'm telling you to do loads of work over the holidays, but the main key is just make sure you are prepared for next term and then have some sleep. The second term in any year is always far busier than any of the others so make sure you do relax and have fun over the holidays.
