The Solar or Lunar Calendar?

Allah says, “They ask you (O Muhammad (pbuh)) about the new moons. Say: These are signs to mark fixed periods of time for mankind and for the pilgrimage” [al-Baqarah 2:189]

Reckoning time by the moon and the new moons was well known to the Prophets and their peoples, and reckoning time by the sun was only known to the ignorant followers of false religions, but unfortunately many Muslims follow them nowadays.

The Scholar, Ibn al-Qayyim (may Allah have mercy on him) said, commenting on the verses “It is He Who made the sun a shining thing and the moon as a light and measured out for it stages” [Yoonus 10:5] and “And the moon, We have measured for it mansions (to traverse) till it returns like the old dried curved date stalk” [YaSeen 36:38, 39]. Hence the lunar calendar was better known among the nations and was less likely to involve errors, and it is more precise than the solar calendar, and all people can follow it easily. Hence Allah says: “and measured out for it stages that you might know the number of years and the reckoning” [Yoonus 10:5]. But He did not say that about the sun, hence the months of Hajj, fasting, Eid and other Islamic occasions are based on the lunar calendar. This is by the wisdom of Allaah and His mercy, and so as to protect His religion, as the people may all be able to figure out this count of time easily and avoid mistakes, and so that no differences or confusion would happen to the religion as happened to the people of the Book.

End quote from Miftaah Daar al-Saadah, p. 538, 539.

It may be understood from the last comment of Ibn al-Qayyim (may Allah have mercy on him) that the people of the Book used to follow a solar calendar, but in fact it played no role in their religion, rather it was introduced after that by the ignorant among them. The Solar or Gregorian calendar was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582 with no religious significance.

Concerning the things we learn from the verse “They ask you (O Muhammad (pbuh)) about the new moons…”, Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
… and we learn that the fixed periods of time for all nations are the fixed periods of time that were established by Allah for them – which are the new moons – so these are global fixed periods of time, because Allah says “fixed periods of time for mankind”. As for what has happened recently, namely the adoption of the European calendar, there it has no tangible basis and it makes no sense and is not prescribed in shareeah. Hence you find that one month has twenty-eight days and some have thirty days and some have thirty one, without any known reason for this discrepancy. Moreover these months have no physical sign that the people can refer to in order to work out time, unlike the lunar months which have a physical sign that is known to everyone.

Tafseer al-Baqarah (2/371).

Al-Qurtubi said, commenting on the verse “Verily, the number of months with Allah is twelve months (in a year), so was it ordained by Allah on the Day when He created the heavens and the earth” [al-Tawbah 9:36]: This verse indicates that rulings on acts of worship and other matters must be connected to the months and years that were known to the Arabs, and not those that were used by the Persians, Byzantines or Copts, and they should not be more than twelve months, because they vary in the number of days; some of them being thirty days and some of them being more or less. But the months of the Arabs never exceed thirty days, although some of them may be less; the ones that are less are not any specific months, rather that varies according to variations in the moon’s movement through the sky. Tafseer al-Qurtubi (8/133).