Muhammad copied Jews and Christians while starting his new religion. He took the stories of monolithic God, prophets and angels from them. But at the same time, he wanted to include Ka'ba and Hajj too in Islam due to the political reasons, as they had immense importance in the Arab World. 

One Arab custom was that 4 months (out of 12) were considered sacred, during which it was not allowed to wage wars or to loot the trading caravans. 

Actually, Arabs started this tradition themselves according to their needs, and then later attributed it to their idol gods that they ordered not to spill the blood and to loot the caravans in those 4 months. 

Moreover, Arabs followed the Lunar Calendar, which had flaws as it had 10 days per year less than the Solar Calendar. This flaw had a huge impact upon the life of Arab people, while they were unable to keep timing of Hajj event and trade caravans always in months of pleasant weather. Due to the Lunar Calendar, Hajj sometimes came in extreme hot weather and sometimes in extreme cold weather. Similarly, many of the trade goods were also attached to the weather situation, and thus they could not be traded in the off season. 

Not only Arabs of that time followed the Lunar Calendar, but Jews also followed the Lunar Calendar too. But Jews realized that flaw of the Lunar Calendar, and thus they brought a reform in the Lunar Calendar. They added one extra month every 3 years to their Lunar Calendar, which made it equal to the Solar Calendar. 

Even the Arabs of time of ignorance also learnt this from the Jews. Thus, in order to keep Haj and trade in pleasant weather, they introduced the reform of postponing the 4 sacred months according to the weather conditions, so that people could easily travel for Hajj and trade. 

Nevertheless, the writer of the Quran (i.e. Muhammad) not only introduced the flawed Lunar Calendar, but he also prohibited the reform of postponing the 4 sacred months. 

(Quran 9:36-37)
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him]. Indeed, the postponing [of restriction within sacred months] is an increase in disbelief by which those who have disbelieved are led [further] astray. They make it lawful one year and unlawful another year to correspond to the number made unlawful by Allah and [thus] make lawful what Allah has made unlawful. 

The Lunar Calendar is non-practical. It has almost no influence upon human life. The whole human life revolves around the Solar Calendar. All crops and goods for trade revolve around the Solar Calendar. 

That is why, although Muslim Scholars insisted a lot upon adopting the Lunar Calendar, but still Muslims left it and they follow the Solar Calendar for their activities. 

Thus, the writer of the Quran is totally wrong when he claims that the whole universe revolves around the 12 Lunar Months. What to talk about the whole universe, these 12 Lunar months are not even important for earth, or even for Muslims. Even Muslims use this Lunar Calendar only for Eid or Ramadhan, otherwise they use Solar Calendar for all the worldly affairs. 

Why did the writer of the Quran declare these 4 months sacred?

Muslims are unable to tell what is special in these 4 Lunar months and why did the writer of the Quran declare them sacred. 

Note, the month of Ramadhan is not included in these 4 sacred months. 

Among these 4 months, 3 come one after another (Dhul Qadha, Dhul Hujjah, Muharram). Trade caravans travelled during these 3 months with peace in the name of Hajj. The 4th month is ‘Rajab’, in which trade caravans travelled in name of ‘Umra’ to Kaba. 

There was no one State present in the area of Arab, but they lived in tribes. And their occupation was looting the trade caravans. Therefore, Arabs declared those 4 months sacred so that they could peacefully perform trade in name of Hajj and Umra to Kaba. 

But as Islam came, then whole Arab came under the control of one state and there was no more looting of trade caravans, and thus those 4 sacred months were no more needed to provide protection to the trade caravans. 

Therefore, a question arose: Why did Allah need to declare only those 4 months sacred in order not to spill blood and not to loot the caravans? 

Common sense guides us, if really there existed any Allah, then he should have given the orders that all 12 months are sacred and no one is allowed to spill blood and to loot the trade caravan throughout the 12 months of the year. 

Such a thing could be considered as God, who tells that people are not allowed for only 4 months, while rest of 8 months they are allowed to wage wars, spill the blood and to loot the trage caravans. 

As a result of Arab becoming one state, there was no war or no trade caravans looting the whole year long. Thus, Muslims of today hardly know about these 4 sacred months and they have no idea why Allah declared them sacred. 

Reality is this that there exist no God, and it was Muhammad himself who followed this custom of the Arabs of time of ignorance. It has nothing to do with these 4 months being sacred, but it was directly connected with the situation of weather in the Arab, and their trade activities. 

Moreover, if any Allah really declared those 4 months sacred since the start of the creation of heavens and the Earth, then it should have also been mentioned in the books of Jews and Christians. Nevertheless, we find nothing in Jewish/Christian books about Hajj or Umra or about 12 Lunar months or 4 sacred months etc. 

Since Jews and Christians didn't have to go out for Hajj once a year, or for Umra, therefore there existed no such thing as 4 sacred months for them, as it existed for the pagans of pre-Islamic era.

Actually, the whole world was following the 12 months lunar Calendar in the era of ignorance. But none of them had any 4 sacred months except of Arab pagans. It is due to the reason while only pagan Arabs had to go to Hajj and Umra once a year. 

Islam Apologists: Allah chose Lunar Calendar in order to break the habits

Islam apologists claim that Lunar Calendar is not flawed, but Allah chose it to break the habits and let people do Hajj in extreme hot and extreme cold, and to fast in extreme cold and extrem hot too. 

But this excuse is useless. Even Allah himself never presented this excuse, but it has been fabricated later by Islam apologists themselves. 

If the repetition of events at same time intervals is really so bad, why then does Nature follow this rule? Why do the 4 seasons repeat themselves at the stipulated time intervals? Why do the crops grow according to the 4 seasons? Why do the birds migrate at the same time of every solar year?

Please note that variations are also present in Nature itself. Days become long or short according to the seasons. Thus, habits break and timings of going to sleep, or getting up, or eating also change according to that. Even times of prayers also change due to that. 

But the repetition in Nature is as necessary as the variation too. Nature brings variations in daily events of life by making days longer or shorter, but when it comes to yearly events, then Nature repeats itself at the same time while crops and trade of goods and whole human life is dependent upon it. 

Harmonizing ourselves with Nature is needed. Whenever we oppose Nature, we will make a mistake. The business of people will be destroyed by opposing Nature. They could not leave their crops standing and go for Hajj. 

The start of “Weekly System” and Islam copying it from earlier cultures

The system of 4 weeks in a month is also a flawed system. A lunar month is equal to 29.53 days and it could not be divided into 4 weeks. 

This system originated in Sumarian culture originally. 

https://www.livescience.com/45432-days-of-the-week.html
The seven-day week originates from the calendar of the Babylonians, which in turn is based on a Sumerian calendar dated to 21st-century B.C. Seven days corresponds to the time it takes for a moon to transition between each phase: full, waning half, new and waxing half. Because the moon cycle is 29.53 days long, the Babylonians would insert one or two days into the final week of each month.

But as Jews declared Saturday a sacred day, similarly Islam declared Friday as a sacred day. In face Muhammad claimed that Friday was also sacred for all the previous nations, but they later distorted it and took other days as sacred ones.

Sahih Muslim, Hadith 856b:
Huraira reported Allah's Messenger (ﷺ) as saying: We were guided aright to Friday (as a day of prayer and meditation), but Allah diverted those who were before us from it. 

Moreover, Muhammad also claimed that Adam was created on the day of Friday, and on this day, he was expelled from paradise, and on Friday he died. 

Sahih Muslim, Hadith 854b:
Abu Huraira reported the Messenger of Allah (ﷺ) as saying: The best day on which the sun has risen is Friday; on it Adam was created. on it he was made to enter Paradise, on it he was expelled from it. And the last hour will take place on no day other than Friday.

Scientific Mistake in Quran regarding 12 Months Lunar Year being the system of Universe

The writer of the Quran (i.e. Muhammad) not only made a mistake in neglecting the Solar Calendar and choosing the Lunar Calendar, but his next mistake was to declare that whole universe is running according to these 12 Lunar months Calendar since the creation of the universe. 

(Quran 9:36-37)
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth

According to modern science, the writer of the Quran is making the following scientific mistakes:

  • The writer of the Quran is claiming that the earth and the universe came into being at the same time. In reality, the earth came into being 9 billion years after the creation of the remaining universe. And the moon came into being even after planet Earth. 
  • Thus, there was no moon present in the heavens (right from the beginning of creation of universe) which could have completed its rotation in 12 months. 
  • And the completion of 12 Lunar months (i.e. 355 days) has no effect on human life, or upon universe/heavens. 

The writer of the Quran (i.e. Muhammad) thought that he was free to fabricate such stories about the heavens and stars as no one is ever able to investigate such stories. 

But Muhammad didn’t know that a time will come when science will be able to investigate his stories and to find out that the was telling lies.