I'd never heard of the expression before, but when I looked it up, I found that it's from a nursery rhyme:
MONDAY'S CHILD IS FAIR OF FACE
by Mother Goose
Monday's child is fair of face,
Tuesday's child is full of grace,
Wednesday's child is full of woe,
Thursday's child has far to go.
Friday's child is loving and giving,
Saturday's child works hard for a living,
But the child born on the Sabbath Day,
Is fair and wise and good and gay.
Maybe most people have heard of this poem and I somehow missed it... But just in case this is new knowledge to anyone else, I thought I'd post it.
What day were you born?