What is reading week?Watch
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Also, some courses don't have reading weeks as far as I know, so check with your individual course before planning anything.
Also, if some don't, some courses will actually make you use that week to work (my Arabic teachers gave us quite a lot of work to do for that week, as opposed to other weeks).
Essentially it gives the academics a week off lectures/marking etc to do other admin - you are expected to keep on working. It certainly shouldnt be regarded as 'holiday'.
However, most people do choose to go home for at least part if the week as it's quite a long time from September to Xmas.