A worked example: February 2026
February 2026 has 28 days, starting on a Sunday and ending on a Saturday — 2026 is not a leap year, so February keeps its standard length.
A 31-day month for contrast: October 2026
October 2026 has 31 days, starting on a Thursday and ending on a Saturday — one of seven months in the Gregorian calendar with 31 days, alongside January, March, May, July, August, and December.
Why month lengths vary at all
The Gregorian calendar's alternating 30-and-31-day pattern (with February as the exception) is a historical convention dating back to the Roman calendar — not a mathematical rule, just an inherited fixed pattern that every Gregorian calendar calculation has to encode correctly.
February's special case
February is the only month whose length genuinely changes year to year — 28 days normally, 29 in a leap year. Every other month has the same fixed length every single year, making February the one case a date calculation has to actively check the year for.
Why the first and last weekday matter for planning
Knowing which weekday a month starts and ends on — not just how many days it has — is useful for scheduling billing cycles, rent due dates, or any recurring plan that depends on a month's exact shape, not just its length.