The Romans named the last day of the week for Saturn, a god they associated with farming. And because the Germanic tribes had no god to substitute for Saturn, many of them kept the Roman name. Thus the Roman dies Saturni developed into the English name Saturday.
The French name samedi may also be a reference to the god of farming. It probably developed from semer, the French word for sowing. But the names used in some languages have a different origin. Saturday is the Jewish day of worship, and in the Bible the seventh day of the week is referred to simply as "Sabbath." Thus Saturday is sabado in Spanish and lordag (Lord's day) in Swedish .
Traditionally, Saturday was bath day. Before homes had indoor plumbing, bath water had to be drawn from a well, carried indoors, and heated over a fire. Once a week was considered quite enough for this work, so every Saturday people cleaned up and got ready to begin a new week. (Grolier New Book of Knowledge Encyclopedia)