Calendar Reference
Where the month and weekday names come from, how the Gregorian leap year rule works, and which years share the same calendar as this one.
Months of the year
| Month | Days | Name origin |
|---|---|---|
| January | 31 | Janus, Roman god of doorways and beginnings |
| February | 28 | Februa, the Roman purification festival · 29 in leap years |
| March | 31 | Mars, god of war — the original first month of the Roman year |
| April | 30 | Latin aperire, "to open" — buds opening in spring |
| May | 31 | Maia, Roman goddess of growth |
| June | 30 | Juno, queen of the Roman gods |
| July | 31 | Julius Caesar, renamed from Quintilis in 44 BC |
| August | 31 | Augustus Caesar, renamed from Sextilis in 8 BC |
| September | 30 | Latin septem, "seven" — the 7th month of the old Roman year |
| October | 31 | Latin octo, "eight" |
| November | 30 | Latin novem, "nine" |
| December | 31 | Latin decem, "ten" |
Days of the week
| Day | Name origin |
|---|---|
| Monday | Moon day — Old English Mōnandæg |
| Tuesday | Týr, Norse god of war (Latin: dies Martis) |
| Wednesday | Odin / Woden (Latin: dies Mercurii) |
| Thursday | Thor, god of thunder (Latin: dies Jovis) |
| Friday | Frigg / Freyja (Latin: dies Veneris) |
| Saturday | Saturn — the only English day still using its Roman name |
| Sunday | Sun day — Old English Sunnandæg |
The leap year rule
A year is a leap year if it is divisible by 4 — except years divisible by 100, which are not — except years divisible by 400, which are. So 2000 was a leap year, 1900 was not, and 2100 will not be. The rule gives an average year of 365.2425 days, within 27 seconds of the tropical year.
The 2026 calendar (not a leap year, starting on a Thursday) is identical to 1981, 1987, 1998, 2009, 2015 and will repeat in 2037, 2043, 2054, 2065, 2071.