:Perfect month
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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.{{Cite web|last=Norris|first=Phil|date=1 February 2021|title=February is looking to be a perfect month on paper|url=https://www.somersetlive.co.uk/news/uk-world-news/february-2021-looking-perfect-month-4951734|access-date=2021-02-01|website=GloucestershireLive|language=en}}{{Cite web|last=de La Taille|first=François|date=31 January 2021|title="Mois parfait", palindrome: quand février 2021 rend fous les fétichistes des chiffres|url=https://www.bfmtv.com/societe/mois-parfait-palindrome-quand-fevrier-2021-rend-fous-les-fetichistes-des-chiffres_AN-202101290185.html|access-date=2021-02-01|website=BFMTV.com|language=fr}} This causes the arrangement of the 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.{{Cite web|last=Shaloo Tiwari|date=1 February 2021|title=Why Is February 2021, a Rectangle Month? Happy Posts Take Over Twitter as the Second Month This Year Forms a Perfect Rectangle Shape!|url=https://www.latestly.com/social-viral/why-is-february-2021-a-rectangle-month-happy-posts-take-over-twitter-as-the-second-month-this-year-forms-a-perfect-rectangle-shape-2296565.html|access-date=2021-02-01|website=latestly.com|language=en}}
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, and 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, 11-6-11, or an 11-11-6 sequence until the end of the 21st century. The most recent perfect months were February 2015 (Sunday-first) and February 2021 (Monday-first).{{Cite web|last=Lombart|first=Gaël|date=30 January 2021|title=Février 2021, mois «parfait»... sur le calendrier|url=https://www.leparisien.fr/societe/fevrier-2021-mois-parfait-sur-le-calendrier-30-01-2021-R7JIJWMI3RCQXH5IMX6ODHQH64.php|access-date=2021-02-01|website=leparisien.fr|language=fr}} Due to calculation rules, the years 1700, 1800, and 1900 are not leap years, causing a shift in the sequence with two instances of three consecutive spacings of six years each (1693, 1699, 1705, 1711 and 1789, 1795, 1801, 1807) and a spacing of twelve years between 1891 and 1903 for Sunday-first calendars; or two consecutive spacings of six years each (1694, 1700, and 1706) and spacings of twelve years between 1790 and 1802, and 1897 and 1909 respectively for Monday-first calendars. The Gregorian calendar repeats every 400 years, so 2094, 2100, and 2106 will all feature perfect months with spacings of six years on Monday-first calendars; while 2093, 2099, 2105, and 2111 will all feature perfect months with spacings of six years on Sunday-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.{{Cite web|last=Focraud|first=Arnaud|date=29 January 2021|title=En février, vous entrerez dans un "mois parfait" (selon le calendrier)|url=https://www.lejdd.fr/Societe/en-fevrier-vous-entrerez-dans-un-mois-parfait-selon-le-calendrier-4021534|access-date=2021-02-01|website=lejdd.fr|language=fr}}{{Cite web|last=Leray|first=Olivia|date=1 February 2021|title=On ne pouvait pas le rater. Malgré le coronavirus, février sera parfait|url=https://www.francetvinfo.fr/replay-radio/on-ne-pouvait-pas-le-rater/on-ne-pouvait-pas-le-rater-malgre-le-coronavirus-fevrier-sera-parfait_4262227.html|access-date=2021-02-01|website=francetvinfo.fr|language=fr}}