Days of the Gods

The Roman week as represented in the Planetary Mosaic

By Conjunto Arqueológico de Itálica

Conjunto Arqueológico de Itálica

Planetary mosaic (Second half of 2nd century AD)Conjunto Arqueológico de Itálica

The Roman Week

The idea of dividing the week into seven days originated somewhere in the Middle East, and is possibly a Jewish concept.

From the 1st century onwards, the Romans began to gradually adopt the seven-day week system. Previously, they had divided the week into eight days, until Emperor Constantine officially announced the seven-day week in the year AD 321.

The names of the days of the week are derived from astrological observations, and they were finalised during the Ptolemaic Egyptian period (around the 1st century BC). Astronomers observed that during the year, most visible stars did not change position in respect to each other, apart from seven celestial bodies: the sun, the moon and the five planets visible to the naked eye: Mars, Mercury, Jupiter, Venus, and Saturn. The days of the week were called after these celestial bodies, which presided over the first few hours of each new day. The order in which the names were given was decided based on an estimate of the distance from these planets and the Earth, giving priority to the Sun—the source of life.

In Itálica we have a splendid work of art representing this system: the Planetary Mosaic. This mosaic paving, dating from the second half of the 2nd century AD, helps us to discover some of the mysteries of the seven-day calendar, its astrological and divine features, and how it evolved to become the system as we know it today.

VR 360 Planetarium HouseConjunto Arqueológico de Itálica

360⁰ video of the Casa del Planetario. Move the mouse to guide the tour.

Sol Invictus in the Planetarium mosaicConjunto Arqueológico de Itálica


Roman tradition considers Sunday, the day of the Sun, to be the first day of the week.

This may have originated from the Jewish tradition, or from the fact that the sun is the prevailing star over the rest of the celestial bodies, or even as a result of the Romans adopting "Sol Invictus" as an official god of the Roman Empire from the 3rd century AD onwards.

What is certain is that it was Emperor Constantine in AD 321 who decreed that the "Dies Solis", i.e. Sunday, was the official day of rest.

The Christian Apostolic tradition dedicated this "Day of the Sun" to rest and to the worshipping of the Lord, calling it "dies Dominicus", or Day of the Lord, a term from which the current name for Sunday is derived in many languages.

In the Planetary Mosaic in Itálica, the Sun is depicted wearing a crown of rays, with long and loose hair, and dressed in a cloak.

Goddess Selene in the Planetarium mosaicConjunto Arqueológico de Itálica


The second day of the Roman week was called "Day of the Moon", our current Monday.
In Saint Isidore's opinion, the Day of the Moon followed the Day of the Sun as a result of the moon receiving light from the sun.

In 1988, the ISO 8601 standard was formulated, an international convention that indicates the order of the days of the week. This rule states that the week starts on Monday and ends on Sunday, constituting the most common order in use today. However, in some liturgical calendars and in some countries, the week still begins on a Sunday.

In the Planetary Mosaic in Itálica, the Moon Goddess is depicted with long, loose hair, easily identifiable thanks to the large crescent moon rising behind her.

God Mars in the Planetarium mosaicConjunto Arqueológico de Itálica


The third day in Roman times was called the Day of Mars, currently Tuesday.

In Roman mythology, Mars was the God of War and was usually depicted wearing armor and a crested helmet. Though associated with the Greek god Ares, Mars is in fact a deity of Italian tradition, patron of many towns and tribes like the Sabines and the Etruscans. Indeed, he was considered the father of Romulus and one of the three tutelary deities of Rome along with Jupiter and Quirinus.

Between the 3rd and 7th centuries, Germanic tribes re-interpreted the Roman tradition, blending Roman deities with their native gods (this is known as the "Germanic interpretation"). This fact is evident in their adopting of the naming system for the days of the week. Thus, in some countries of Germanic tradition, Tuesday derives from Tiw (or Tyr), the Germanic war deity that was assimilated with the Roman Mars. This is the case, for example, for the English use of Tuesday.

In the Planetary Mosaic of Itálica, Mars is depicted with a beard and wearing the characteristic crested helmet and armor.

God Mercurius in the Planetarium mosaicConjunto Arqueológico de Itálica


The fourth day of the Roman week, Wednesday, was dedicated to the god Mercury.

The planet Mercury was originally observed by Sumerians and subsequently by the Babylonians. They identified this planet with the god Nabu (or Nebo), God of Literature and Wisdom, traits that were subsequently equated with the Greek god Hermes and then to his Roman equivalent, Mercury.

The representation of Mercury in the Planetary Mosaic of Itálica shows a figure of a young man with wings on his head, symbols of his speed in fulfilling his duties as messenger of the gods.

God Jupiter in the Planetarium mosaicConjunto Arqueológico de Itálica


Thursday, the fifth day of the Roman week, was called the Day of Jupiter.

For a time, Jupiter was the chief god in Roman mythology, protector of justice and law. He borrowed various characteristics from the Greek Zeus, such as his pre-eminent position over the rest of the gods, as well as his role as the bearer of light.

As we saw in the case of Tuesday, for Germanic tribes the Roman god Jupiter was identified with the native god Thor, God of Thunder. From that, the name "Day of Thor" was used in some countries of Germanic tradition, as is the case with the English use of Thursday.

In the Planetary Mosaic of Itálica, Jupiter is depicted with a beard, wearing a cloak that covers only his shoulders and a laurel wreath, a symbol of grandeur.

Goddess Venus in the Planetarium mosaicConjunto Arqueológico de Itálica


The sixth day of the Roman week, Friday, was dedicated to the goddess Venus.

Because of its orbit and its position relative to the Earth, the planet Venus is visible only during the first three hours after sunset and the last three hours before sunrise. For this reason, it has been given the names "Morning Star" and "Evening Star." As a divinity, Venus is related to love, beauty, and fertility.

For this reason, the Day of Venus in the Roman week was adopted by the Germanic peoples as the Day of Frigg, since she was the Goddess of Fertility, Love, The Home, Marriage, Motherhood and the Domestic Arts. In addition, the Nordic name for the planet Venus was Friggjarstjarna, "star of Frigg." This is, for example, the etymological origin of the English Friday.

Venus is the central character in the Planetary Mosaic in Itálica. It is thought that this central position could derive from her role as the protector of marriage. She is depicted with a precious stone adorning her neck and wearing a crown—elements that are, perhaps, linked to the distinctive brightness of this star.

God Saturn in the Planetarium mosaicConjunto Arqueológico de Itálica


Saturn's day, Saturday, is the last day of the Roman week.

Of all the planets that are visible to the naked eye from planet Earth, Saturn is the most distant and has the slowest orbit. In fact, Saturn takes almost 30 years (29.457 years) to complete its course, nearly three times longer than Jupiter. For this reason, the planet and Roman god of Jupiter was equated with the Greek god Zeus, while Saturn was associated with his father, the Titan Cronus.

However, the current name of Saturday, which has the same root in most European languages, comes not from the Roman tradition but rather from the Hebrew term "shabat", or "Sabbath", meaning "Day of Rest." It's interesting how, in this case, English has retained the Latin etymology, with the word "Saturday."

Saturn is represented in the Planetary Mosaic at Itálica as a male figure with a thick beard, symbolizing the passage of time. His head is covered with a veil, typical attire of the officiator during religious rituals in Ancient Rome.

Credits: Story

Days of the Gods

Organised by:
Conjunto Arqueológico de Itálica
Consejería de Cultura de la Junta de Andalucía

Texts: Daniel González Acuña
Cover Photography: Francisco José Marín Fatuarte
Detailed Photographies: Daniel González Acuña

Conjunto Arqueológico de Itálica.

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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