If you could make do with a basic calendar, with one month on each page of A4 (so that you could write your shift alongside each date), you've probably already got one on your PC. Well, if you've got Microsoft Word you should have.
Go to File > New > New from Template > General Templates > Other Documents > Calendar Wizard. (That route might vary if your version of Word is different to mine but you should find a calendar wizard in there somewhere).
Alternatively, if you'd prefer to have a 'proper' diary, any decent stationers will sell you one. They should all have loads of them right now because it's the start of a new year. No, I've not just recovered from a 7-month hangover after the New Year's Eve celebrations, I'm referring to
academic year diaries. They're available in both pocket and desk sizes; they typically run from August of one year through to September of the following year.
As stated any stationery shop should stock them, or you can buy online:
http://www.amazon.co.uk/Collins-2008-09-Academ ic-150x80mm-Assorted/dp/B001230XUG/ref=pd_sbs_ ce_3
Chris