Days are faithful as days go. They start at midnight between one and the other and end 24 hours exactly later. Very faithful. Completely fidelic. Oh, but yes, once every few score years the most careful timekeeping men and women must perforce add a leap second, for the Earth is slowing in its spin, like a figure skater on one tiptoe once she launches each spin she slows. But still, few things more faithful than a day to its begining and thence to its end, where the next takes over. Each 24 hours of 60 minutes each.
But some days are more FATEFUL than others. One day a baby is born. Another day the baby is grown into an man and marries a woman, or a woman who marries a man. And then one day they have their own child, and another day another child. And then one very very fateful day, that person dies. So a few days are very fateful in a person's life.