The defining moment of each day of your life is the moment you do something for the first time. Most days have moments like that, even if you don't realise it at the time.
Now, I'm not talking about deliberately doing daft things like standing on one leg and shouting "HERMAPHRODITE!" just for the sake of making a dull day original. I'm talking about the tiny, tiny things that happen naturally in the course of your day, like walkng down a new street, trying a new food, talking to a new person, finding out something you didn't know before. Then you can look back on the day and think, Oh yes, that was the day I sat in chewing gum in my lunch break.
That was yesterday. Today will be the day I sat on my bed with a pre-addressed envelope and a plastic vial and sacrificed my body in the name of scientific research. Well, some of my DNA on the end of a stick anyway. I'm pretty sure I've never done that before.