Editing Month (section)

=== Roman calendar ===
The [[Roman calendar]] was reformed several times, the last three enduring reforms during historical times. The last three reformed Roman calendars are called the ''Julian'', ''Augustan'', and ''Gregorian''; all had the same number of days in their months. Despite other attempts, the names of the months after the [[Augustan calendar]] reform have persisted, and the number of days in each month (except February) have remained constant since before the [[Julian calendar|Julian reform]]. The [[Gregorian calendar]], like the [[Julian calendar|Roman calendars]] before it, has twelve months, whose [[Anglicized]] names are:
<!--DO NOT abbreviate the names of the months! There is plenty of room for their full names in English. Thanks.--> 
:{| class="wikitable sortable"
|- style="vertical-align:bottom;"
! Order !! Name !! Number<br>of days 
|-  style="text-align:center;"
|  1 
|style="text-align:left;"| '''[[January]]''' || 31 
|-  style="text-align:center;"
|  2 
|style="text-align:left;"| '''[[February]]'''
| 28<br/>{{small|29 in [[leap year]]s}}
|-  style="text-align:center;"
|  3 
|style="text-align:left;"| '''[[March]]''' || 31
|-  style="text-align:center;"
|  4 
|style="text-align:left;"| '''[[April]]''' || 30
|-  style="text-align:center;"
|  5 
|style="text-align:left;"| '''[[May]]''' || 31
|-  style="text-align:center;"
|  6 
|style="text-align:left;"| '''[[June]]''' || 30
|- a style="text-align:center;"
|  7 
|style="text-align:left;"| '''[[July]]'''<br/>{{small|formerly ''[[Quinctilis]]''}}
| 31
|-  style="text-align:center;"
|  8 
|style="text-align:left;"| '''[[August]]'''<br/>{{small|formerly ''[[Sextilis]]''}}
| 31
|-  style="text-align:center;"
|  9 
|style="text-align:left;"| '''[[September]]''' || 30 
|-  style="text-align:center;"
| 10 
|style="text-align:left;"| '''[[October]]''' || 31
|-  style="text-align:center;"
| 11 
|style="text-align:left;"| '''[[November]]''' || 30
|-  style="text-align:center;"
| 12 
|style="text-align:left;"| '''[[December]]''' || 31
|}

{{Self reference|One of Wikipedia's sister projects, [[Wiktionary]], provides translations of each of the Gregorian/Julian calendar months into a dozen or more languages. Month-by-month links are provided here: [[wikt:January|January]], [[wikt:February|February]], [[wikt:March|March]], [[wikt:April|April]], [[wikt:May|May]], [[wikt:June|June]], [[wikt:July|July]], [[wikt:August|August]], [[wikt:September|September]], [[wikt:October|October]], [[wikt:November|November]], [[wikt:December|December]].}}

[[File:Month - Knuckles (en).svg|thumb|300px|On top of the knuckles (yellow): 31&nbsp;days<br/>Between the knuckles (blue): 30&nbsp;days<br/>February (red) has 28 or 29&nbsp;days.]]
[[File:Klaviatur-3-en.svg|thumb|300px|The white keys of the musical keyboard correspond to months with 31&nbsp;day months. ('''F''' corresponds to January.)]]
The famous [[mnemonic]] ''[[Thirty days hath September]]'' is a common way of teaching the lengths of the months in the English-speaking world. The knuckles of the four fingers of one's hand and the spaces between them can be used to remember the lengths of the months. By making a fist, each month will be listed as one proceeds across the hand. All months landing on a knuckle are 31&nbsp;days long and those landing between them are 30&nbsp;days long, with variable February being the remembered exception. When the knuckle of the index finger is reached (July), go over to the first knuckle on the other fist, held next to the first (or go back to the first knuckle) and continue with August. This physical mnemonic has been taught to primary school students for many decades, if not centuries.<ref>{{cite web |year=1997 |title=Days in each month |website=Mnemonics to Improve Memory |publisher=EUdesign |url=http://www.eudesign.com/mnems/dayspcm.htm |access-date=8 August 2018}}</ref><ref>{{cite book |title=The Boy Mechanic: A handy calendar |year=1913 |volume=1 |publisher=[[Project Gutenberg]] |via=Full Books |url=http://www.fullbooks.com/The-Boy-Mechanic-Volume-17.html}}</ref>

This cyclical pattern of month lengths matches the [[musical keyboard]] alternation of wide white keys (31&nbsp;days) and narrow black keys (30&nbsp;days). The note '''F''' corresponds to '''January''', the note '''F{{sup|{{music|#}}}}''' corresponds to '''February''', the exceptional 28–29&nbsp;day month, and so on.

==== Numerical relations ====
The mean month-length in the Gregorian calendar is 30.436875&nbsp;days.

Any five consecutive months that do not include February contain 153&nbsp;days.

==== Calends, nones, and ides ====
Months in the pre-Julian [[Roman calendar]] included:
* ''[[Intercalaris]]'' an [[intercalary month]] occasionally embedded into February, to realign the calendar.
* ''[[Quintilis]]'', later renamed to ''Julius'' in honour of [[Julius Caesar]].
* ''[[Sextilis]]'', later renamed to ''Augustus'' in honour of [[Augustus]].

The Romans divided their months into three parts, which they called the ''calends'', the ''nones'', and the ''ides''. Their system is somewhat intricate. 
The '''[[ides (calendar)|ides]]''' occur on the thirteenth day in eight of the months, but in March, May, July, and October, they occur on the fifteenth. The '''[[Roman calendar#Months|nones]]''' always occur 8&nbsp;days (one Roman 'week') before the ides, i.e., on the fifth or the seventh. The '''[[calends]]''' are always the first day of the month,{{efn|More precisely, the [[calends]] were when the name of a month first began being used when referring to dates. Instead of counting the number of days elapsed, the Romans used a [[countdown]] to number their dates. See the article ''[[Roman calendar]]'' for a more detailed explanation.}} and before Julius Caesar's reform fell sixteen days (two Roman weeks) after the ides (except the ides of February and the intercalary month).

==== Relations between dates, weekdays, and months in the Gregorian calendar ====
Within a month, the following dates fall on the same day of the week:

* '''01, 08, 15, 22, and 29''' (e.g., in January 2022, all these dates fell on a Saturday)
* '''02, 09, 16, 23, and 30''' (e.g., in January 2022, all these dates fell on a Sunday)
* '''03, 10, 17, 24, and 31''' (e.g., in January 2022, all these dates fell on a Monday)
* '''04, 11, 18, and 25''' (e.g., in January 2022, all these dates fell on a Tuesday)
* '''05, 12, 19, and 26''' (e.g., in January 2022, all these dates fell on a Wednesday)
* '''06, 13, 20, and 27''' (e.g., in January 2022, all these dates fell on a Thursday)
* '''07, 14, 21, and 28''' (e.g., in January 2022, all these dates fell on a Friday)

Some months have the same date/weekday structure.

In a non-leap year:

* '''January/October''' (e.g., in 2022, they began on a Saturday)
* '''February/March/November''' (e.g., in 2022, they began on a Tuesday)
* '''April/July''' (e.g., in 2022, they began on a Friday) 
* '''September/December''' (e.g., in 2022, they began on a Thursday)
*  '''1 January''' and '''31 December''' fall on the same weekday (e.g. in 2022 on a Saturday)

In a leap year:

* '''February/August''' (e.g., in 2020, they began on a Saturday)
* '''March/November''' (e.g., in 2020, they began on a Sunday)
* '''January/April/July''' (e.g., in 2020, they began on a Wednesday)
* '''September/December''' (e.g., in 2020, they began on a Tuesday)
* '''29 February''' (the leap day) falls on the same weekday like 1, 8, 15, 22 February and 1 August (see above; e.g. in 2020 on a Saturday)