Until the eighteenth century Saturn was considered the last and farthest planet, the full stop to creation's great sentence. As Milton put it:
Here Nature first begins
Her farthest verge and chaos to retire,
As from her outmost works a broken foe,
With tumult less and with less hostile din.
Just as Jupiter was the father of Mercury, Venus and Mars; so Saturn was the father of Jupiter. And in direct contravention of all the rules of good parenting he used to try and kill all his kiddies by eating them. Jupiter, however, avoided such digestion, overthrew his father and imprisoned him. That is why Saturn wanders such a long and melancholy track, and why those born under his influence are miserable and saturnine.
This is particularly so on Saturn's special day, Saturday, when everybody is unhappy. As Saturn is the god of lead (Mars=iron Venus=copper), saturnism is lead poisoning.