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!