They are the origins of the names of each day of the week.

Sunday – Sun day.
Monday – Moon day.
Tuesday – Tiw’s or Týr’s day, the god of single combat, son of Odin.
Wednesday – Woden’s or Odin’s day, god of poetry and of the dead.
Thursday – Thor’s day, god of thunder, son of Odin.
Friday – Frigg’s day, goddess of the clouds, wife of Odin.
Saturday – Saturn’s day, god of fertility and agriculture.

Put seven pigs in the first three pens and arrange the fourth pen to contain the other three pens.