Where Month and Weekday Names Come From

The names of the months come from ancient Rome. Six honor gods or festivals, two honor rulers, and four are simply Latin numbers left over from a year that began in March. The names of the weekdays are older still: they follow the seven “planets” of ancient astronomy, and English swapped four of the Roman gods for Germanic ones.

That split explains the most common question about calendar names: why September, whose name means seven, is the ninth month.

The Roman year that started in March

Roman tradition credited the city’s founder, Romulus, with a year of only ten months. As described in Smith’s Dictionary of Greek and Roman Antiquities, that year had 304 days: four months of 31 days (Martius, Maius, Quinctilis and October) and six of 30. The arithmetic checks out: 4 × 31 + 6 × 30 = 124 + 180 = 304. Historians treat the ten-month year as legend, but the month names themselves preserve its order.

Count from March and the numbered names line up exactly:

Position in the old year	Latin name	Meaning	Position today
1	Martius	Mars	3 (March)
2	Aprilis	uncertain	4 (April)
3	Maius	Maia	5 (May)
4	Iunius	Juno	6 (June)
5	Quintilis	fifth	7 (July)
6	Sextilis	sixth	8 (August)
7	September	seven	9
8	October	eight	10
9	November	nine	11
10	December	ten	12

Every numbered month now sits two places later than its name says, because two months were later placed in front of March.

Why September is no longer the seventh month

Tradition says Numa Pompilius, Rome’s second king, added January and February. According to the Roman writer Censorinus, as summarized in Smith’s dictionary, Numa’s year had 355 days, with January at 29 days and February at 28, and every month except February had an odd number of days. One way that adds up: the four old 31-day months, seven months of 29 days and a 28-day February give 124 + 203 + 28 = 355.

Adding months did not by itself move the start of the year. A practical change did. Livy’s summary of events records that Rome’s consuls began entering office on January 1, a change prompted by a revolt in Hispania, and the date is usually given as 153 BC. Before that, consuls had taken office on March 15. With the official year beginning on January 1, January and February became months 1 and 2, and March slid to third.

Julius Caesar’s calendar reform, which took effect in 45 BC, kept January 1 as the start of the year. The numbered names never changed, so September (7) is month 9, October (8) is month 10, November (9) is month 11 and December (10) is month 12.

Where each month name comes from

• January is named for Janus, the Roman god of beginnings and transitions.
• February most likely comes from the februa, Roman purification rites held in that month. The link is widely accepted but not certain.
• March honors Mars, the Roman god of war, and was the first month of the old year.
• April has no settled origin. Two leading ideas are a link to the Etruscan name Apru and the Latin verb aperire, “to open.”
• May is usually traced to Maia, a Roman goddess.
• June is usually traced to the goddess Juno, though ancient writers offered other explanations.
• July was Quintilis, “fifth month,” until it was renamed Julius in 44 BC for Julius Caesar, who was born in that month.
• August was Sextilis, “sixth month,” until it was renamed for the emperor Augustus. The renaming is usually dated to 8 BC.
• September, October, November and December are the Latin numbers seven, eight, nine and ten.

Renamings that did not stick

Other emperors tried to put their names on the calendar. The Roman biographer Suetonius records that Nero called April “Neroneus.” Domitian renamed September “Germanicus” and October “Domitianus,” explaining that he had become emperor in the first and been born in the second. After Domitian’s death the old names came back, which is why only Caesar and Augustus kept their months.

The planets behind the seven weekday names

The weekday names follow the seven bodies that ancient astronomers saw moving against the stars: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn. In Latin, the days were dies Solis, dies Lunae, dies Martis, dies Mercurii, dies Iovis, dies Veneris and dies Saturni.

The Roman historian Cassius Dio, who wrote in Greek, said the custom of naming days after the planets came from Egypt and was comparatively recent. He gave two explanations without choosing between them. The second, based on planetary hours, can be checked with simple arithmetic.

Worked example: the planetary-hour rule

Dio’s rule assigns each hour of the day and night to a planet, cycling through them in this order: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Each day takes the name of the planet that rules its first hour.

1. Give the first hour of day 1 to Saturn. Day 1 is Saturn’s day.
2. A day has 24 hours. Since 24 = 3 × 7 + 3, after 24 hours the cycle has run three times and moved three places further. Three places after Saturn (Jupiter, Mars, Sun) is the Sun. Day 2 is the Sun’s day.
3. Three places after the Sun (Venus, Mercury, Moon) is the Moon. Day 3 is the Moon’s day.
4. Keep stepping three places: Mars, then Mercury, then Jupiter, then Venus, and then back to Saturn.

The result is Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus: Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday. That is exactly the order of the modern week.

How English swapped in Germanic gods

When Germanic-speaking peoples adopted the Roman week, they translated the names, replacing four Roman gods with the closest match from their own beliefs. Old English kept the result, and modern English kept nearly all of it.

Day	Old English	God or body	Latin equivalent
Sunday	Sunnandæg	the Sun	dies Solis
Monday	Monandæg	the Moon	dies Lunae
Tuesday	Tiwesdæg	Tiw (Tyr)	dies Martis (Mars)
Wednesday	Wodnesdæg	Woden (Odin)	dies Mercurii (Mercury)
Thursday	Þunresdæg	Thunor (Thor)	dies Iovis (Jupiter)
Friday	Frigedæg	Frige	dies Veneris (Venus)
Saturday	Sæternesdæg	Saturn	dies Saturni

Saturday is the odd one out: English kept the Roman god’s name rather than substituting a Germanic one.

Languages descended from Latin kept the Roman gods instead. French uses lundi, mardi, mercredi, jeudi and vendredi for Monday through Friday, and Spanish uses lunes, martes, miércoles, jueves and viernes. Set them beside the Latin names and Mars, Mercury, Jupiter and Venus are easy to spot.

Portuguese went another way and numbers most of its weekdays: segunda-feira (Monday) through sexta-feira (Friday), while keeping sábado and domingo for the weekend.

Reading old dates by their names

The month names are fixed, but their positions are not guaranteed in every source. A Roman text that mentions Quintilis or Sextilis is talking about the months we call July and August, and a document dated by an emperor’s renamed month, such as Domitian’s Germanicus, means September.

Weekday names give you a built-in check. A letter or diary that gives both a weekday and a date can be tested against the calendar: the day of the week calculator shows whether the two agree, and the guide to finding the day of the week by hand explains the method. For dates before the 1750s, also check whether the writer was using the Julian or Gregorian calendar, covered in the Gregorian calendar history. The ISO week numbers guide covers why some calendars start the week on Monday and others on Sunday, and any year calendar puts all twelve month names and seven weekday names on one page.