Allah Azza wa Jall says in the Qur’aan Kareem “It is He who gave the sun radiance and the moon light, and determined the stages that you may learn the calculationof years and reckoning of time. Allah created all this with a rightful purpose. He makes His signs clear for those who know”. Surah Younus, verse 5.
The Islamic calendar also known as the Hijri calendar (abbreviated as A.H meaning anno hegirae) was established in about 638 CE (christian era) by the second Khaleef (Caliph), Sayyidinaa Umar bin al Khattaab (RA) in order to have a common calendar with countries under the Islamic state, such as Misr (Egypt), Faarsi (Persia), and Shaam (Syria). These countries had their ancient non-Islamic calendars based on solar phases with 365 days but each had a different starting point.
The Pre Islamic Persian calendar began with June 16, 632 CE – the date of the accession of the last sassanid ruler King Tezdgird III.
Pre-Islamic Syria being part of the Byzantine Empire, used a form of the Roman Empire, which began October 31, 312 BC (that’s three hundred and twelve years before Eesaa AS , way before the Persian calendar!)
Pre-Islamic Egypt used the Coptic calendar starting at August 29, 284 CE.
Sayyidinaa Umar (RA) chose the day of Nabi Muhammad’s (SAW) hijrah (migration) from Makkah to Madeenah where he (SAW) passed away ten years later. The Hijrah took place on Muharram 1, 1 A.H (July 16, 622 CE).
Since the Islamic calendar is based on lunar phases which has 354 days, eleven less than the Gregorian solar year, it is therefore not synchronised to the seasons. Hence festivals such as Eid,etc will fall on the same lunar (Islamic) months each year but different seasons as the years progress.
by Mu’allimah Sa’uda