The clocks change shortly, which brings us to this fascinating piece of information. Islam forbids ‘intercalation’, which Wikipedia says is:
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require a combination of both adjustments.
In the Julian Calendar as well as in the Gregorian Calendar that improved it, intercalation is done by adding an extra day to February in each leap year. In the Julian Calendar this was done every 4 years. In the Gregorian calendar years whose number is evenly divisible by 100 but not 400, were exempted in order to improve accuracy.
The solar year does not have a whole number of lunar months either, so a lunisolar calendar must have a variable number of months in a year. This is usually 12 months, but sometimes a 13th month (an intercalary or embolismic month) is added to the year.
ISO 8601 includes a specification for a 52-week year. Any year that has 53 Thursdays has 53 weeks; this extra week may be regarded as intercalary.
Here is a page that explains it further in the context of Islam:
The number of months with Allah has been twelve months by Allah’s ordinance since the day He created the heavens and the earth. Of these four are known as forbidden [to fight in]; That is the straight usage, so do not wrong yourselves therein, and fight those who go astray. But know that Allah is with those who restrain themselves.
Verily the transposing (of a prohibited month) is an addition to Unbelief: The Unbelievers are led to wrong thereby: for they make it lawful one year, and forbidden another year, of months forbidden by Allah and make such forbidden ones lawful. The evil of their course seems pleasing to them. But Allah guideth not those who reject Faith.
O People, intercalation is an addition to unbelief, through it [God, Allah] leads the unbelievers astray: they make it permissible one year and forbid it [at their mere convenience] the next one to elude the timing of what God forbade, so that they make permissible that which Allah forbade [fighting in the forbidden months], and forbid that which Allah has made permissible [fighting in other months]. And [now, this year] time has turned the way it was the day God created Heavens and Earth [The intercalary months since the creation of Heavens and Earth have all canceled out (summed up to whole years)]. The year is twelve months, four of them are forbidden, three successive: Dhu al-Qi’dah and Dhu al-Hijjah and Muharram; and the Rajab of Mudar which is between Jumada and Shaban.
The three successive forbidden months mentioned by Muhammad (months in which battling is forbidden) are Dhu al-Qi’dah, Dhu al-Hijjah, and Muharram, thus excluding an intercalary month before Muharram. The single forbidden month is Rajab. These months were considered forbidden both within the new Islamic calendar and within the old pagan Meccan calendar, although whether they maintained their “forbidden” status after the conquest of Mecca has been disputed among Islamic scholars.
File under, “you learn something every day!”.