What is the answer to the riddle "How many months have 28 days?"

All 12 months! Every month of the year has at least 28 days.

How many days are in February during a leap year?

February has 29 days in leap years and 28 days in common years.

How does the ISO-8601 standard determine whether a calendar year has 52 or 53 weeks?

Under the ISO-8601 international standard, a calendar year contains 53 weeks if January 1 falls on a Thursday in a common year, or on either a Wednesday or Thursday in a leap year. In all other years, the calendar contains exactly 52 weeks (364 days), leaving 1 or 2 residual days that carry over into the adjacent year's ISO week cycle.

Why do solstices and equinoxes shift calendar dates between June 20-22 and December 20-23?

The Gregorian calendar standardizes a 365-day common year, but Earth takes approximately 365.2422 solar days to complete one tropical orbit around the Sun. This ~6-hour annual discrepancy causes astronomical solar events (solstices and equinoxes) to drift roughly 6 hours later each year until the 4-year leap day reset pulls them backward by 24 hours.

How Many Months Have 28 Days (Calendar Math & Trivia)

The Logic & History of 28 Days in Calendar Months

The question "How many months have 28 days?" is one of the world's most famous calendar riddles. People instinctively answer "One" (thinking of February). But mathematically, all 12 months have at least 28 days:

Seven months have 31 days (which includes 28 days + 3 days).

Four months have 30 days (which includes 28 days + 2 days).

February has 28 days in common years, or 29 days in leap years.

Why Does February Have Fewer Days?

In the 8th century BCE under Roman King Numa Pompilius, the Roman calendar had 10 months plus winter months. The Romans considered even numbers unlucky, so months had 29 or 31 days. To fit a 355-day lunar year, February was assigned 28 days as the end-of-year purification month (Februa). When Julius Caesar reformed the calendar in 45 BCE into 365 days, February was kept at 28 days with a leap day every 4 years.

The Mathematics of Calendar Chronometry & ISO-8601 Standards

Civil time reckoning relies on the Gregorian calendar, a solar calendar established in 1582 to rectify historical drift between civil dates and tropical vernal equinoxes. Modern digital computing systems and enterprise enterprise resource planning (ERP) platforms build upon this baseline by implementing international standards like ISO-8601, which codifies unambiguous calendar representations, ordinal day tracking, and standardized week numeration.

1. Ordinal Day of the Year (DOY) Formulation

An ordinal date represents a calendar date by specifying the four-digit Gregorian year and an integer ordinal day ($DOY$) ranging from $1$ (January 1st) to $365$ (December 31st in common years) or $366$ (December 31st in leap years). Formally, the ordinal day for month $M \in [1, 12]$ and day $D$ is computed as:

// Mathematical Ordinal Day Formula

DOY = D + ∑ (Days in Month m) for m = 1 to M - 1

Where Month Days = [31, 28 + L, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

Leap indicator L = 1 if (Y mod 4 == 0 and Y mod 100 != 0) or (Y mod 400 == 0) else 0

2. ISO-8601 Week Numbering & Boundary Overlap Rules

Unlike traditional civil weeks which may begin on Sunday, the ISO-8601 standard strictly defines Monday as Day 1 and Sunday as Day 7. Under ISO-8601 clause 2.2.10, Week 01 of any given year is defined as the week that contains the first Thursday of that year (equivalent to the week containing January 4th). This mathematical definition produces critical operational rules:

Majority Rule: Week 01 always contains at least four days of the new calendar year.
Boundary Shifting: If January 1st falls on a Friday, Saturday, or Sunday, those days belong to Week 52 or Week 53 of the preceding year. Conversely, if December 29, 30, or 31 falls on a Monday, Tuesday, or Wednesday, those days belong to Week 01 of the subsequent year.
Leap Week Years (53 Weeks): Common years beginning on a Thursday, or leap years beginning on a Wednesday or Thursday, contain 53 ISO weeks instead of the customary 52.

3. Astronomical Solar Mechanics: Solstices, Equinoxes & Daylight Variance

The cycle of calendar seasons is governed by Earth's axial obliquity (currently tilted at ~23.44° relative to its orbital plane) and its elliptical orbit around the Sun. At the Summer Solstice (~June 20–21 in the Northern Hemisphere), the North Pole attains maximum axial tilt toward the Sun, producing the longest period of daylight and the shortest solar night. Six months later, at the Winter Solstice (~December 21–22), the Northern Hemisphere reaches minimum solar irradiance, producing the shortest day and longest night.

How Many Months Have 28 Days (Calendar Math & Trivia)