It entirely depends on the point of the diary. Are you going to write it in advance, so that you can look at it and see what you're going to be doing on any specific day? Or are you going to write it in arrears, like a record of what you did?
If it's in advance, I would say that you just need a simple layout like the one shown in the previous response:
Thursday May 20th 2010
- Buy food
- Send email to Dave
- Get hair cut
- Job interview 2pm at Green House, High Street.
That's more like a "To Do" list, and might not help your English writing
though!
However, if you're going to "write a diary" as children (and adults!) are sometimes advised to do, as a sort of therapeutic exercise, then you would write in whole sentences and paragraphs.
Thursday May 20th 2010
I woke up feeling out of sorts, though I didn't know why. I didn't sleep very well last night and had bad dreams, so maybe that's the reason. Once I'd had breakfast and been for a short walk in the park, I felt much happier and ready to face the day. The argument I had at work last week was still preying on my mind and I really need to let that go. I met Peter in town and we had a great lunch at a new restaurant - I must take my sister there next time she visits - she'll love it.
As you can see, that would be much better for your writing practice!