as is my custom...

I came in on a rain cloud. Chances are, i'll go out on a rain cloud as well.
One reason I just thought of to celebrate someone's (or your own) birthday is that extravagantly rich people who spend frivilously make every day their birthday. Therefore, you deserve one day to claim as your own. Everyone was born, so everyone should have a day to enjoy each year. We're in competition with those richies. You know how the saying goes...keeping up with the jones'. Besides, this way, you value aging in a much wiser fashion then those who make every day their birthday. Possessions obtained, pictures drawn by children for you, and any other trinkets received have much higher value. Such sentimental value is something no one can ever take away from you.
Do a raindance in the hope the rivers don't flood too badly.