Saturnus, Caravaggio, 16th c.

Saturday (IPAEng: ˈsætɚdeɪ, ˈsætɚdi, ˈsætədeɪ, ˈsætədi Listen) is the day of the week between Friday and Sunday. Saturday is considered either the sixth or seventh day of the week.

It was named no later than the second century for the planet (Saturn), which controlled the first hour of that day according to Vettius Valens. The planet was named for the Roman god of agriculture Saturn. It has been called dies Saturni ("Saturn's Day"), through which form it entered into Old English as Sæternesdæg and gradually evolved into the word "Saturday".

Saturday is the only day of the week in which the English name comes from Roman mythology. The English names of all of the other days of the week come from Germanic polytheism. In India, Saturday is Shanivar, based on Shani, the Vedic God manifested in the planet Saturn. In the Thai solar calendar of Thailand, the day is named from the Pali word for Saturn, and the color associated with Saturday is purple. The Celtic languages also name this day for Saturn: Irish an Satharn or dia Sathuirn, Scottish Gaelic Disathairne, Welsh dydd Sadwrn, Breton Sadorn or disadorn.

In Jewish Law Saturday is the seventh day Shabbat. Thus, in many languages the Saturday is named after the Sabbath. Eastern Orthodox churches distinguish between the Sabbath (Saturday) and the Lord's Day (Sunday). Roman Catholics put so little emphasis on that distinction that many among them follow – at least in colloquial language – the Protestant practice of calling Sunday the sabbath (see Sabbath in Christianity). Quakers traditionally refer to Saturday as "Seventh Day" eschewing the "pagan" origin of the name. In Islamic countries, Fridays are holidays, however they are considered as the sixth day of the week.

In the Eastern Orthodox Church Saturdays are days on which the Theotokos (Mother of God) and All Saints are commemorated, The day is also a general day of prayer for the dead, because it was on a Saturday that Jesus lay dead in the tomb. The Octoechos contains hymns on these themes, arranged in an eight-week cycle, that are chanted on Saturdays throughout the year. At the end of services on Saturday, the dismissal begins with the words: "May Christ our True God, through the intercessions of his most-pure Mother, of the holy, glorious and right victorious Martyrs, of our reverend and God-bearing Fathers…". For the Orthodox, Saturday is never a strict fast day. When a Saturday falls during one of the fasting seasons (Great Lent, Nativity Fast, Apostles' Fast, Dormition Fast) the fasting rules are always lessened to an extent. The Great Feast of the Exaltation of the Cross and the Beheading of St. John the Baptist are normally observed as strict fast days, but if they fall on a Saturday or Sunday, the fast is lessened.

In Scandinavian countries, Saturday is called Lördag or Laurdag, the name being derived from the old word laugr/laug (hence Icelandic name Laugardagur), meaning bath, thus Lördag equates to bath-day. This is due to the Viking usage of bathing on Saturdays.