As a calendar maker, I am quite interested in calendar folklore and history, including the different start dates for the year. Many calendars have their origins in the agricultural cycle and thus begin the year in spring, with the vernal equinox, or in the fall, with the harvest season. Our calendar’s start date has its origins in the Roman practice of beginning the year shortly after the winter solstice, when the days begin to lengthen again. All of these traditions are based on the sun and what is called the solar year.
Then there’s the Muslim calendar, which begins the new year on August 20 this time — but really this evening (August 19), because an Islamic day begins at sunset. Next year it will occur on Aug. 10, and in 2022 it moves into July. It will keep moving forward in the Gregorian calendar because the Islamic calendar is strictly lunar, without adjustments to make it align with the solar year.
Whereas the traditional Jewish and Chinese calendars, and a few others, are luni-solar — a hybrid that inserts an extra (intercalary) month or two at regular intervals so that holidays move forward on the Gregorian calendar (the lunar part), but then move back again so as to stay in the same season (the solar part) — the Islamic calendar makes no such accommodation, perhaps because its basis is religious, not agricultural. Prior to that, the nomadic Arab people had been using a lunar calendar, which was adapted by Muslims to mark their founding event.
The Islamic calendar began in the common year 622 (AD/CE) and the month of Muharram, when Muhammad and his followers migrated from Mecca, where they were persecuted, to Medina, where they established the first Muslim community. The migration is known as the Hijri, which is also the name of the Islamic calendar.
The new year is called al-Hijri (or al-Hijra) or Muharram 1. (The different spellings reflect different transliterations from the Arabic.)
Addendum for nerds like me
This page started as a Facebook post on my business page, but quickly became unwieldy for that format. The information is based broadly on the various books and articles I have read over the dozen or so years I have been fascinated by calendars. But I did not wish to take the time to go back through my sources and notes to cite anything specific, so do take the foregoing with a grain of salt. I did check Wikipedia to refresh my memory and get a couple of details straight, though.
For those who want more information, the Wikipedia page on the Hijri year offers a nice concise explanation.
For a broad history of calendars presented in an engaging format with photos and illustrations, visit the web museum Calendars through the Ages.
These two sources don’t entirely agree on the details, and neither is reliable enough to use alone for fact-checking or term paper research, but they do provide some historical and cultural context for understanding the Islamic calendar (and others, in the second one).
I welcome corrections from those who know more than me about this topic, and other relevant comments. But I do moderate so I can delete the spammy stuff.