There are many interesting facts about the Hijri calendar such as:

v     The Hijri calendar conforms to the Text of the Glorious Qur’ān, and the aḥādith.  In the blessing sūrah  of at-tawbah (9) we find out:

إِنَّ عِدَّةَ الشُّهُورِ عِنْدَ اللَّهِ اثْنَا عَشَرَ شَهْرًا فِي كِتَابِ اللَّهِ يَوْمَ خَلَقَ السَّمَاوَاتِ وَالأرْضَ …(٣٦)

36.  Verily, the number of months (in a year) with Allāh has been twelve in the Book[ii] of Allāh.  It was so ordained by Him the day when He created the heavens and the earth…

v     The number of months (emphatically at 12 in the Glorious Qur’ān) corresponds to the number of infallible ’imāms, i.e. 12.

v     The position of moon (and stars) to the sun, i.e. reflecting its light to all corners of the world, is very similar to the position of infallible ’imāms to the Prophet.  They reflect the radiance of his guidance (هداية) to the hearts of all people, everywhere in the world.

v     As the movement of the moon brings about some physical phenomena such as ebb and flow, it also brings about some spiritual feelings in the hearts of people.  Those who do not observe the lunar calendar are unaware of these internal hidden sensations.

v     The hidden characteristic of each hijri month is quite unique and very significant.  Because of this, special precepts are recommended for certain times throughout the hijri calendar for spiritual uplift.  From physical and spiritual points of view, not all months, weeks, days, and hours are the same.  They have specific hidden secrets, characters, properties, and qualities according to the movements of the sun and the moon.  In a hijri calendar Muslims take opportunity of all good moments for appropriate spiritual communications, supplications, and support.  

v    There are tens of reliable aḥādith stating that the inner meaning of the word an-Najm (the Star), and that of the word ʿAlāmāt (Signs) in the blessing Sūrah of An-Naḥl (16), refer to prophet Muḥammad (ph&hp) and the Ahl al-Bayt (a.s.), respectively.  We know that the moon and the stars reflect the light of the sun at night when it is dark.  Some specific stars (Signs) have been used as the traditional navigating guides for caravan leaders in the desert, and for ship captains in the ocean, to find their safe and secure destination.  By the same token, Prophet Muḥammad (The Star النّجم) and the rest of the Wilāyah (Signs علامات) are spiritual guides for all people of the world in the bewildered darkness of ignorance.  They receive the radiance of spiritual guidance from the very One Source (Allāh (J.J.)) and propagate it to all people of the world, as the moon and the stars take their physical light and heat from the very one source (the Sun) and enlighten all corners of the world, in the darkness of night.  In Sūrah an-Naḥl (16) we read:

وَ أَلْقَى‏ فىِ الأَرْضِ رَوَاسىِ‏َ أَن تَمِيدَ بِكُمْ وَ أَنهَْارًا وَ سُبُلًا لَّعَلَّكُمْ تهَتَدُونَ(15)  وَ عَلامَاتٍ ^ج وَبِالنَّجْمِ هُمْ يَهْتَدُونَ (١٦)

15-16.  And He has cast firm Mountains in the earth, lest it may shake you, and rivers and paths, so that you may be guided and (also Specific) Signs; And they themselves are guided by the Star.

v    The importance of the light of guidance of prophet Muḥammad (ph&hp) and that of the Wilāyah (a.s.), and the similarity of this radiance to the reflection of the celestial bodies (the stars and the moon) is important enough to allow for the allocation of one long chapter in book number 3.  Please refer to book 3, v. 24, chapter 30 for more information regarding all aḥādith relevant to this subject.  

v    The lunar calendar is a vivid proof and evidence of the popular phrase in Islamic literature: “Every day is ʿĀshūrā and every land is Karbalā[iii].”  We observe that the ʿĀshūrā is not fixed to a certain time of the year.  It can be in spring, summer, autumn, or winter because of the flowing nature of the lunar calendar.  The same thing is true with all important events of Islam, such as the Ḥajj, Mabʿath, Hijrah, and birth dates of prophet Muḥammad  and the infallible ’imāms.  In other words, all these events are beyond any season of the year; they flow year-round in the lunar calendar.  Conversely, all events become rigidly fixed to a very one point in all solar calendars, and fail to render fluidity, timelessness, and permanence character.  Therefore, in the world of Islam, Muslims celebrate all holy dates, such as ʿid al-Qurbān (al-’Aḍhā),  ʿid al-Fiṭr, ʿid al- Mabʿath, perform certain acts of worship, such as the Ḥajj and other pilgrimages, and conduct special prayers, fasting and mourning, in all seasons of the year, as times goes on.  Apart from the “time” element caused by the lunar calendar, the events of the world are also vivid proofs and evidences of the above-mentioned phrase.  Severe ẓulm (oppression) is not limited to the very location of Karbalā and the specific time of ʿĀshūrā .  Look at the extreme atrocity in Iraq, Afghanistan, Bosnia, Palestine, Lebanon, Kashmir, Ghazza (Gaza), Yemen, and many other places in the world in different locations and at different times.  So, every day is a new ʿĀshūrā, and every place is a new Karbalā.

v    Working with the hijri calendar, makes us familiar and friendly with the heavenly sky and its beauty, with the wisdom of the continuous movements of celestial bodies, and with the magnanimity of its amazing creation.  Looking at the sky and pondering its nature has been the source of guidance, reflection, spiritual energy, and pleasure for all prophets and messengers of Allāh  and is considered as an act of worship:

إِنَّ فىِ خَلْقِ السَّمَاوَاتِ وَ الْأَرْضِ وَ اخْتِلَافِ الَّيْلِ وَ النهَّارِ لاَيَاتٍ لّأِوْلىِ الْأَلْبَابِ (190)

Indeed in the creation of the heavens and the earth and the alternation of night and day, there are signs for those who possess intellects (190).

Sūrah  Āl i-ʿImrān (3)

أَ فَلَمْ يَنظُرُواْ إِلىَ السَّمَاءِ فَوْقَهُمْ كَيْفَ بَنَيْنَاهَا وَ زَيَّنَّاهَا وَ مَا لهَا مِن فُرُوجٍ(6)

Have they not then observed the sky above them, how We have built it and adorned it, and that there are no cracks in it?(6)

Sūrah  Qāf (50)

v    Muslims observe movements of the moon, as well as the sun, in their day-to-day living.  Timing of all daily ritual practices are based on the movement of sun for determination of the exact time of sunrise, dawn, religious midday, sundown, eventide (evening adhān), and religious midnight.  The Hijri lunar calendar, however, is used for other purposes throughout the year.  Muslims have to be watchful of the sky for the movements of sun, moon, and other stars and Signs, day and night.  In fact, Islam is known to be unique in reaping the benefits of applying astronomy in day-to-day religious matters.  Finding the direction of Makkah and the time of Ṣalāt are the reasons which led to Muslims developing spherical geometry.

v    This is why Islamic scientists such as Abū Alī Sīnā (Avicenna), Rāzi, Khwārizmī, al-Farghānī, al-Battānī, Fārābī, ʿUmar Khayyām, and many others, were ahead of all others in the science of astronomy and the associated sciences such as mathematics, physics, trigonometric, algebra (al-Jabr), geometry, etc.  Whatever the European scientists learned about it later, was based on the outcome of what Muslim scientists had attained before.  Calculation of huge astronomical numbers with utmost accuracy is the proof of their mastery in the science of numerics, as an example.  The next section, “MUSLIMS ASTRONOMERS” clarifies this subject.  The most complete, accurate, and logical set of calendars, that have perfect compatibility with the divine creation of the heavens and earth, belong to the Muslims, either the solar or the lunar.

v    Allāh (J.J.), the creator and decorator of the heavenly sky has commanded human beings to look at the beautiful sky and observe its structure and the way it is built.  Not only have we ignored, these days, to pay particular attention to the sky as a religious act of worship, but also have ruined this heavenly source of grace and blessing by creation of all possible pollutions, to the point that the sky cannot be even seen in large cities.   

v    On the basis of the following Āyah, Muslims began to develop better observational and navigational instruments; thus, most navigational stars today have Islamic names.

وَ هُوَ الَّذِى جَعَلَ لَكُمُ النُّجُومَ لِتهَْتَدُواْ بهِا فىِ ظُلُمَاتِ الْبرَّ وَ الْبَحْرِ  قَدْ فَصَّلْنَا الاَيَاتِ لِقَوْمٍ يَعْلَمُونَ (97).

It is He who made the stars for you, so that you may be guided by them in the darkness of land and sea.  We have certainly elaborated the Signs for those who have knowledge.

Sūrah Al-’Anʿām (6), Āyah 97

v    Other influences of the Glorious Qur’ān on Islamic astronomy included its insistence that the Universe is ruled by a single set of Divine Laws which is deeply rooted in the Islamic principle of tawḥīd (unity), the unity of Allāh (j.j.) , and all His created beings.  Another Islamic emphasis was on greater respect for empirical data than was common among the preceding Greek philosophers.