A member14 in official Page on facebook has asked this question. Answer needed as early.
Why Islamic calendar based on lunar system?
Publicly posted. Hence privacy disclosed. https://www.facebook.com/SHAFI.CPWD
The question has been closed for the following reason "The question is answered, right answer was accepted" by Irfan Alam Apr 06 at 06:20
[6:96] At the crack of dawn, He causes the morning to emerge. He made the night still, and He rendered the sun and the moon to serve as calculation devices. Such is the design of the Almighty, the Omniscient.
Specifically, the phases of the moon are cited as a timing device for the people in Quran 2:189 and 10:5.
[2:189] They ask you about the phases of the moon! Say, "They provide a timing device for the people, and determine the time of Hajj. ...
[10:5] He is the One who rendered the sun radiant, and the moon a light, and He designed its phases that you may learn to count the years and to calculate. GOD did not create all this, except for a specific purpose. He explains the revelations for people who know.
God makes it very clear in the Quran that there are twelve months. We are also told that four of these months are sacred (months 12, 1, 2, 3). While some muslims alternate the sacred and regular months, God condemns this in Quran 9:37. Thus we know that all the four sacred months are sequential (i.e. one after the other).
God's System: Twelve Months Per Year [9:36] The count of months, as far as GOD is concerned, is twelve. This has been GOD's law, since the day He created the heavens and the earth. Four of them are sacred. This is the perfect religion; you shall not wrong your souls (by fighting) during the Sacred Months.
This should be proof enough that it was Allah that legislated it. It cannot be confused with the liner calendar. If more proof is needed than your welcome to ask.
Bismillah. The reason is that in surah al-Baqarah Allah says "They ask thee concerning the New Moons. Say: They are but signs to mark fixed periods of time in (the affairs of) men, and for Pilgrimage". So it is just fitting to use the lunar system. Oh yeah by the the way the source is 2:189. Hope I answered your question. Salam.