Gregorian Calendar 2026 — History, 12 Months, Leap Year Explained

fact_checkKey Facts

• check_circleIntroduced by Pope Gregory XIII in 1582
• check_circleConsists of 12 solar months
• check_circleA year = 365 or 366 days (leap year)
• check_circleOfficially used in most countries worldwide
• check_circleLeap year occurs every 4 years — February has 29 days

history_eduHistory of the Gregorian Calendar — Who Created It?

Pope Gregory XIII introduced the Gregorian calendar on October 4, 1582, replacing the Julian calendar established by Julius Caesar in 45 BCE. The primary motivation was to correct the Julian calendar's accumulated error — it added a leap day every 4 years without exception, causing it to drift roughly one day every 128 years relative to the solar year. By 1582, the calendar had drifted about 10 days. The Gregorian reform removed 10 days and introduced the century-year exception for leap years. Catholic countries adopted it quickly; Protestant and Orthodox countries followed over the next few centuries, with the last adopters in the 20th century.

wb_sunnyHow Does the Gregorian Calendar Work?

The Gregorian calendar is based on Earth's orbit around the Sun — the tropical year — which is approximately 365.2425 days. To handle the fractional days, a leap day (February 29) is added roughly every 4 years. This keeps calendar dates aligned with the seasons. Because it tracks the solar year so precisely, the seasons always occur in the same months: spring in March–May, summer in June–August, autumn in September–November, and winter in December–February (in the Northern Hemisphere). The calendar is divided into 12 months of varying lengths: 28–31 days each.

event_repeatThe Leap Year Rule

A leap year in the Gregorian calendar has 366 days — an extra day is added as February 29. The rule: (1) A year divisible by 4 is a leap year (e.g., 2024, 2028). (2) Exception: Century years (e.g., 1800, 1900, 2100) are only leap years if divisible by 400 (e.g., 2000 was a leap year; 1900 was not). This three-rule system keeps the calendar accurate to within 1 day every ~3,300 years. Without leap years, the calendar would drift about 1 month every 120 years.

calendar_view_monthThe 12 Months

1. 1
   January(31 days)

   First month of the year

2. 2
   February(28 or 29 days)

   Shortest month — 29 days in leap years only

3. 3
   March(31 days)

   Spring equinox (~March 20) in the Northern Hemisphere

4. 4
   April(30 days)
5. 5
   May(31 days)
6. 6
   June(30 days)

   Summer solstice (~June 21) in the Northern Hemisphere

7. 7
   July(31 days)

   Named after Julius Caesar

8. 8
   August(31 days)

   Named after Emperor Augustus

9. 9
   September(30 days)

   Autumn equinox (~September 23) in the Northern Hemisphere

10. 10
    October(31 days)
11. 11
    November(30 days)
12. 12
    December(31 days)

    Last month of the year — winter solstice (~December 21)

compare_arrowsComparison: Hijri vs. Gregorian