The Islamic calendar is a lunar calendar that plays an important role in the sanctification of time for Muslims. In order to gain a better understanding, it is helpful to understand the structure of the calendar and what distinguishes it from other calendars in use throughout history. To that end, we note that the Islamic calendar has three integral elements.

First, there are twelve lunar months in which every month is determined by the arrival of the new moon. These months are: Muḥarram (the first month of the year), Ṣafar, Rabīʿ al-Awwal, Rabīʿ al-Thānī, Jumādá al-Ūlá, Jumādá al-Ukhra, Rajab, Shaʿbān, Ramaḍān, Shawwāl, Dhū al-Qaʿdah, and Dhū al-Ḥijjah.

In his exegesis of the Qur’an, al-Jāmiʿ li-aḥkām al-Qurʾān, Allamah Qurṭubī (RA) states that Allah decreed and determined the twelve months, the day He created the heavens and the earth, established and named these months, and revealed them to the Noble Prophets in their revealed books. The names and order of these months existed before Islam but were tampered with over time; however, it was according to Allah’s divine will that they would be recalibrated by the time the Prophet ﷺ gave his Farewell Sermon. The Prophet ﷺ said:

عَنْ أَبِي بَكْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ “‏ إِنَّ الزَّمَانَ قَدِ اسْتَدَارَ كَهَيْئَتِهِ يَوْمَ خَلَقَ اللَّهُ السَّمَوَاتِ وَالأَرْضَ،
“Time has come back to its original state which it had when Allah created the heavens and the earth.” [Sahih al-Bukhari 4662]
In addition, the Islamic lunar calendar runs ten to eleven days behind the Gregorian solar calendar yearly and as a result does not synchronize with the seasons, a feature that distinguishes it from the lunisolar Hebrew (Jewish) calendar, which offsets the difference every two or three years by inserting a 13th leap month to synchronize the lunar months with the seasons.

Second, the Islamic calendar contains four sacred months: Muḥarram, Rajab, Dhū al-Qaʿdah, and Dhū al-Ḥijjah. The Prophet ﷺ said (in the same Hadith as above):

السَّنَةُ اثْنَا عَشَرَ شَهْرًا مِنْهَا، أَرْبَعَةٌ حُرُمٌ، ثَلاَثٌ مُتَوَالِيَاتٌ، ذُو الْقَعْدَةِ وَذُو الْحِجَّةِ وَالْمُحَرَّمُ وَرَجَبُ مُضَرَ الَّذِي بَيْنَ جُمَادَى وَشَعْبَانَ
The year is twelve months, four of which are sacred. Three of them are in succession; Dhul-Qa’da, Dhul-Hijja and Al-Muharram, and (the fourth being) Rajab Mudar (named after the tribe of Mudar as they used to respect this month) which stands between Jumad (ath-thani) and Sha’ban.

Lastly, it is a marker of the migration (Hijrah) of Prophet Muḥammad ﷺ from Mecca to Medina in 622 CE, which marks the beginning of the Islamic era, and is a fixed point from which the first year of the calendar (1 AH) has been determined. Thus, another name given to it is the Hijrī calendar.