📆 Calendar Reference
Essential calendar constants: month lengths, abbreviations, fiscal quarters, astronomical seasons, and leap year rules.
Months of the Year
| # | Month | Abbr. | Days |
|---|---|---|---|
| 1 | January | Jan | 31 |
| 2 | February | Feb | 28 |
| 3 | March | Mar | 31 |
| 4 | April | Apr | 30 |
| 5 | May | May | 31 |
| 6 | June | Jun | 30 |
| 7 | July | Jul | 31 |
| 8 | August | Aug | 31 |
| 9 | September | Sep | 30 |
| 10 | October | Oct | 31 |
| 11 | November | Nov | 30 |
| 12 | December | Dec | 31 |
Calendar Quarters (Calendar Year)
| Quarter | Months | Days | Date Range |
|---|---|---|---|
| Q1 | January – March | 90 days (91 leap) | Jan 1 – Mar 31 |
| Q2 | April – June | 91 days | Apr 1 – Jun 30 |
| Q3 | July – September | 92 days | Jul 1 – Sep 30 |
| Q4 | October – December | 92 days | Oct 1 – Dec 31 |
Astronomical Seasons (Northern Hemisphere)
| Season | Start | Approx. End |
|---|---|---|
| Spring | March 20 (Vernal Equinox) | June 20 |
| Summer | June 21 (Summer Solstice) | September 21 |
| Fall | September 22 (Autumnal Equinox) | December 20 |
| Winter | December 21 (Winter Solstice) | March 19 |
Dates are approximate and vary by ±1 day year to year. Southern hemisphere seasons are reversed.
Leap Year Rule
A year is a leap year if it is divisible by 4, except century years (divisible by 100), which must also be divisible by 400 to be leap years.
- ✓ 2024 — leap (divisible by 4)
- ✓ 2000 — leap (divisible by 400)
- ✗ 1900 — not a leap year (divisible by 100 but not 400)
- ✓ 2028 — leap year (next after 2024)