Yes, by all measures a birthday is merely the anniversary of the Earth attaining the same position in space around it's parent star as it did when a person was born. Nothing too special right? Takes place every year.
What makes this astronomically mundane event special, though, is that it can be used as an excuse to celebrate relations that one holds. From your entire family getting together to celebrate it, to calls from friends you haven't contacted for quite a while. It's a day that will have everyone expressing warmth and good wishes, making it truly one's own day.
And birthday cakes. Birthday cakes make it a special day.