The Muslim New Year.
In between my 12 hour day job and 7 hour sleep time I get about a couple hours to let my little brain roam free. Sometimes this can be a little dangerous. Why? Because I tend to muse on subjects which are of no economic benefit. Which means it's not going to help put food on the table. However there is one particular topic which keeps bugging me every year. It's about the Muslim calendar.
Yes, that Islamic calendar that most of us lay Muslims know about, use regularly, but hardly know how it came about.
Ya, ya I knew the second Khalifah Umar Bin Khatab (RA) had it drawn up and used extensive throughout the Islamic world by the year 638 CE. But by that time it was already 16 years after the Hijrah or the Journey that the Prophet (SAW) made from Mecca to Medinah. Which means it was already the Muslim year 16 A.H.(Anno Hegirae, Latin for 'in the year of the Hijrah').
So Umar's administration did not actually invent or start the use of the Islamic calendar. The people of that area were already using lunar months to indicate dates well before the Prophet's time. What Umar actually did was to standardise and rationalise the various, and at times conflicting, dating systems used during his time. He did this by fixing the starting point for indicating actual dates and start numbering the years.
So Muharram 1, 1 A.H. or the first day of the Islamic calendar was calculated back to correspond to July 16, 622 C.E. Many people confuse this date as being the date of the Migration of the Prophet from Mecca to Medinah. From Hadiths or Traditions the day that the Prophet arrived in Medinah from Mecca was 12 Rabie Al Awal which corresponds to September 23, 622 C.E. For conversions between C.E.(Gregorian) and A.H.(Hijrah) dates click http://www.islamicity.com/PrayerTimes/convert.asp.
So what's the significance of all this, you may ask. Nothing much really except it gave me a chance to read up on things. You get a few side information like the earliest date of Islamic calendar for which a Julian (Gregorian) calendar date is exactly known is 9th Dhul-Hijjah, 10 AH, which corresponds to March 6, 632 C.E. (Friday), when the Prophet Muhammad (SAW) performed his last and farewell pilgrimage to Mecca.

Refs:
http://en.wikipedia.org/wiki/Hijri_year
http://www.al-islam.org/al-serat/hijrah.htm
http://www.salaam.co.uk/themeofthemonth/june02_index.php?l=2