The seventh day of the week in many religious traditions, and the sixth day of the week in systems using the ISO 8601 norm; observed as the Sabbath/Shabbat in Judaism (Friday sunset to Saturday sunset); it follows Friday and precedes Sunday.
"After observing the Sabbath from Friday sunset until now, we know that today is the seventh day of the week known as Saturday."
In plain English: Saturday is the seventh day of the week, usually coming after Friday and before Sunday when most people have time off from work or school.
"We usually go to church on Saturday morning."
On Saturday.
"We plan to have a barbecue on Saturday afternoon."
Saturday comes from Old English, originally meaning "Saturn's day" as a direct translation of the Latin phrase for the same concept. The name traveled into English by associating the day with the Roman god Saturn rather than using an older Germanic deity.