Pick a day between Sunday, Monday, Wednesday, Thursday, Friday, and Saturday, and you can say that one seventh of your life was that day.
But how much of your life was a Tuesday?
For those who don't get it and want the joke spoiled for themselves:
[spoiler]https://www.bungie.net/en/Forum/Post/218401575/0/0/1[/spoiler]
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That's not necessarily true. If I was born on a Friday and died on a Friday, I lived most of my life on a Friday. A shade over 1/7th of my life was on a Friday while the rest of the days got a shade under, depending on how long I lived.