The Gregorian calendar is the calendar used in most of the world. It is named after Pope Gregory XIII, who introduced it in October 1582.
The calendar was developed as a correction to the Julian calendar, shortening the average year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.
The Gregorian calendar continued to employ the Julian months, which have Latinate names and irregular numbers of days:
- January (31 days), from Latin mēnsis Iānuārius, "Month of Janus", the Roman god of gates, doorways, beginnings and endings
- February (28 days in common and 29 in leap years), from Latin mēnsis Februārius, "Month of the Februa", the Roman festival of purgation and purification,cognate with fever, the Etruscan death god Februus ("Purifier"),[citation needed] and the Proto-Indo-European word for sulfur
- March (31 days), from Latin mēnsis Mārtius, "Month of Mars", the Roman war god[48]
- April (30 days), from Latin mēnsis Aprīlis, of uncertain meaning but usually derived from some form of the verb aperire ("to open") or the name of the goddess Aphrodite
- May (31 days), from Latin mēnsis Māius, "Month of Maia", a Roman vegetation goddess whose name is cognate with Latin magnus ("great") and English major
- June (30 days), from Latin mēnsis Iūnius, "Month of Juno", the Roman goddess of marriage, childbirth, and rule
- July (31 days), from Latin mēnsis Iūlius, "Month of Julius Caesar", the month of Caesar's birth, instituted in 44 bc as part of his calendrical reforms
- August (31 days), from Latin mēnsis Augustus, "Month of Augustus", instituted by Augustus in 8 bc in agreement with July and from the occurrence during the month of several important events during his rise to power
- September (30 days), from Latin mēnsis september, "seventh month", of the ten-month Roman year of Romulus c. 750 bc
- October (31 days), from Latin mēnsis octōber, "eighth month", of the ten-month Roman year of Romulus c. 750 bc
- November (30 days), from Latin mēnsis november, "ninth month", of the ten-month Roman year of Romulus c. 750 bc
- December (31 days), from Latin mēnsis december, "tenth month", of the ten-month Roman year of Romulus c. 750 bc
Europeans sometimes attempt to remember the number of days in each month by memorizing some form of the traditional verse "Thirty Days Hath September". It appears in Latin, Italian,and French, and belongs to a broad oral tradition but the earliest currently attested form of the poem is the English marginalia inserted into a calendar of saints c. 1425:
Thirty days have November,
April, June, and September.
Of 28 is but one
And all the remnant 30 and 1.
