I remember the first time I realized that the day I was living would be the only time that day ever happened. I can't remember how old I was exactly; I was likely between 9 and 11 years old since I remember what house I lived in when this occurred. I was playing Barbies and listening to the radio, and something they said on the radio just triggered it. I stopped and realized that the particular day was going to be the only October 12, 1993...ever (not the specific date but used for the purpose of this entry). It was a little earth-shattering for me. For some reason, I had never really thought of life as being a finite thing and that days were not repeated or recycled. It also fascinated me. Those who were born after that date would never get to experience that date, just as I would never experience dates that happened before I was born.
I'd like to say that this gave me a profound revelation to live each day to the fullest or take charge of life, but it didn't. I sat there for a few minutes, then happily went back to my Barbies. If there was only going to be one October 12, 1993, I was going to live it how I wanted to.