Perfect month

A perfect month or a rectangular month designates a month whose number of days is divisible by the number of days in a week and whose first day corresponds to the first day of the week. This causes arrangement of days of the month to resemble a rectangle. In the Gregorian calendar, this arrangement can only occur for the month of February.

Constraints
To satisfy such an arrangement in the Gregorian calendar, the number of days in the month must be divisible by seven. Only the month of February of a common year can meet this constraint as the month has 28 days, a multiple of 7.

For a February to be a perfect month, the month must start on the first day of the week (usually considered to be Sunday or Monday). For Sunday-first calendars, this means that the year must start on a Thursday. As for Monday-first calendars, the year must start on a Friday. It must also occur in a common year, as the phenomenon does not occur when February has 29 days.

Occurrence
In the Gregorian calendar, the phenomenon occurs every six years or eleven years following a 6-11-11 sequence until the end of the 21st century. The most recent perfect months were February 2015 (Sunday-first) and February 2021 (Monday-first). Due to calculation rules, the years 1800 and 1900 are not leap years, causing a shift in the sequence with a spacing of twelve years between 1790 and 1802 and between 1897 and 1909 respectively; however 2094, 2100 and 2106 will all feature perfect months with spacings of six years on Monday-first calendars.

The next perfect months will be February 2026 (Sunday-first) and February 2027 (Monday-first).

Attributes
The calendar arrangement brings together notions of harmony and organization.

Related articles

 * Perfect months: February 1909, February 1915, February 1926, February 1937, February 1943, February 1954, February 1965, February 1971, February 1982, February 1993, February 1999, February 2010, February 2021
 * Palindrome § Dates
 * Perfectionism (psychology)
 * Perfectionism (philosophy)