It happens. Three days before is actually not that bad. I'd say if you wanted you could take an evening off/have a drink/meal or something...not write the entire day off, but spend a bit of time off working.
I had GCSE exams on my birthday, which was a pretty rubbish. Last year 1st year vetmed exams started the day after my birthday - so I did nothing for it. My flatmates were all so stressed/busy revising as well that they forgot and then remembered later and found a random cupcake in the kitchen and put a candle in it and sang happy birthday outside my room, and that was all I managed birthday related all day. Two weeks later there were several of us who had a belated birthday party!
So, yeh, you're not the only one by far!