With regard to the postponement of the months mentioned in the verse (interpretation of the meaning): “The postponing (of a Sacred Month) is indeed an addition to disbelief: thereby the disbelievers are led astray, for they make it lawful one year and forbid it another year in order to adjust the number of months forbidden by Allah, and make such forbidden ones lawful. The evil of their deeds seems pleasing to them. And Allah guides not the people, who disbelieve” [at-Tawbah 9:37], this refers to their practice whereby they would decide that the sacred month of Muharram was not to be held sacred, and they would declare Safar as sacred instead, if they needed to do that.
This has been discussed previously in the answer to question no. 165970.
To sum up what has been mentioned in some books of history:
Before Islam, the Arabs used to use the lunar calendar, in which there were twelve months that were determined by the sighting of the new moon from one month to the next.
Among them there were four sacred months, during which they would refrain from fighting; they would hold their markets, such as ‘Ukaaz and others; they would go on pilgrimage to the Ka‘bah; and they would be safe on the road and at home from raids, plunder and banditry.
But despite the fact that they used the lunar calendar, they did not have any particular system for dating important events, in most cases. Rather, when it came to dating important life events, they would connect them to certain historical events, as follows:
·The construction of the Ka‘bah by Ibraaheem al-Khaleel and his son Ismaa‘eel (peace be upon them both) – approximately 1855 BCE
·The Year of Treason, which was the year in which Banu Yarboo‘ stole the goods that some of the kings of Banu Himyar had sent to the Ka‘bah – 461 BCE
·The collapse of the Ma’rib Dam in Yemen – 120 BCE approx.
·The death of Ka‘b ibn Lu‘ayy, the seventh great-grandfather of the Messenger Muhammad (blessings and peace of Allah be upon him) – 59 BCE
·The Year of the Elephant, which is the year in which the great Messenger Muhammad (blessings and peace of Allah be upon him) was born – 571 CE
·The Sacrilegious War, which was so called because the Arabs committed a grave act of sacrilege in it, when their tribes fought amongst themselves during the sacred months. This war lasted for four years, beginning in 586 CE.
·The reconstruction of the Ka‘bah, which occurred at the time of ‘Abd al-Muttalib, the grandfather of the Messenger Muhammad (blessings and peace of Allah be upon him). At that time, the Messenger (blessings and peace of Allah be upon him) was 35 years old. This means that it occurred in 605 CE, i.e., five years before the mission of Muhammad (blessings and peace of Allah be upon him) began.
Throughout their long history before Islam, the Arabs used the names of the lunar months that were known at that time, until these names changed and throughout the Arab land began to take on the forms by which they are known now, from the end of the fifth century CE – during the era of Kilaab, the fifth great-grandfather of the Messenger Muhammad (blessings and peace of Allah be upon him), as was stated by al-Biruni in 412 CE. During the Jaahiliyyah, the Arabs also used the solar months during some periods and in some places.
For more information, please see: al-Mufassal fi Tareekh al-‘Arab qabl al-Islam (1/5014); Murooj adh-Dhahab by al-Mas‘oodi (1/249)
And Allah knows best.