In the Name of Allah, Most Merciful, Most Compassionate
I pray that you are well.
The Islamic year has twelve months that are based on a lunar cycle, namely: Muharram, Safar, Rabia Awal, Rabia Thani, Jumaada Awal, Jumaada Thani, Rajab, Sha'ban, Ramadhan, Shawwal, Dhul-Qi'da and Dhul-hijjah.
Four of these months are sacred.
Abu Bakrah, may Allah be pleased with him, reported that the Prophet, Allah bless him and grant him peace, said:
The year is twelve months of which four are sacred, the three consecutive months of Dhu’l-Qi’dah, Dhul-Hijjah and Muharram, and Rajab Mudar which comes between Jumaada and Sha’ban.[al-Bukhari]
The Prophet, Allah bless him and grant him peace, during his address of the Day of Sacrifice during the Last Hajj, explained the sacred months by saying: The months are consecutive - Dhul-Qi'dah, Dhul-Hijjah, Muharram - and one month is that of Rajab. But, there were two sayings of the Arabs regarding the month of Rajab. Some tribes used to call the month we know as Ramadhan the month of Rajab while, as seen by the tribe of Mudar, Rajab was the month which comes in between Jumada ath- Thaniah and Sha'ban. Therefore, the Holy Prophet, Allah bless him and grant him peace, by mentioning this month as 'Rajab Mudar' - also made it clear that it means the month of Rajab which is in between Jumada ath-Thaniah and Sha'ban. [tafsir of Qur'an -- Ma'ariful Qur'an by Mufti Muhammad Shafi]
There is no dispute that Dhul-hijjah being one of the four sacred months.
Allah Almighty says,
إِنَّ عِدَّةَ ٱلشُّهُورِ عِندَ ٱللَّـهِ ٱثْنَا عَشَرَ شَهْرًا فِى كِتَـٰبِ ٱللَّـهِ يَوْمَ خَلَقَ ٱلسَّمَـٰوَٰتِ وَٱلْأَرْضَ مِنْهَآ أَرْبَعَةٌ حُرُمٌ ۚ ذَٰلِكَ ٱلدِّينُ ٱلْقَيِّمُ ۚ فَلَا تَظْلِمُوا۟ فِيهِنَّ أَنفُسَكُمْ ۚ وَقَـٰتِلُوا۟ ٱلْمُشْرِكِينَ كَآفَّةً كَمَا يُقَـٰتِلُونَكُمْ كَآفَّةً ۚ وَٱعْلَمُوٓا۟ أَنَّ ٱللَّـهَ مَعَ ٱلْمُتَّقِينَ ﴿٣٦[Qur'an At-Tawba 9:36]
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 therein. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him]. (36)
يَسْـَٔلُونَكَ عَنِ ٱلشَّهْرِ ٱلْحَرَامِ قِتَالٍ فِيهِ ۖ قُلْ قِتَالٌ فِيهِ كَبِيرٌ ۖ وَصَدٌّ عَن سَبِيلِ ٱللَّـهِ وَكُفْرٌۢ بِهِۦ وَٱلْمَسْجِدِ ٱلْحَرَامِ وَإِخْرَاجُ أَهْلِهِۦ مِنْهُ أَكْبَرُ عِندَ ٱللَّـهِ ۚ وَٱلْفِتْنَةُ أَكْبَرُ مِنَ ٱلْقَتْلِ ۗ وَلَا يَزَالُونَ يُقَـٰتِلُونَكُمْ حَتَّىٰ يَرُدُّوكُمْ عَن دِينِكُمْ إِنِ ٱسْتَطَـٰعُوا۟ ۚ وَمَن يَرْتَدِدْ مِنكُمْ عَن دِينِهِۦ فَيَمُتْ وَهُوَ كَافِرٌ فَأُو۟لَـٰٓئِكَ حَبِطَتْ أَعْمَـٰلُهُمْ فِى ٱلدُّنْيَا وَٱلْـَٔاخِرَةِ ۖ وَأُو۟لَـٰٓئِكَ أَصْحَـٰبُ ٱلنَّارِ ۖ هُمْ فِيهَا خَـٰلِدُونَ ﴿٢١٧[Qur'an Al-Baqarah 2:217]
They ask you about the sacred month - about fighting therein. Say, "Fighting therein is great [sin], but averting [people] from the way of Allah and disbelief in Him and [preventing access to] al-Masjid al-Haram and the expulsion of its people therefrom are greater [evil] in the sight of Allah. And fitnah is greater than killing." And they will continue to fight you until they turn you back from your religion if they are able. And whoever of you reverts from his religion [to disbelief] and dies while he is a disbeliever - for those, their deeds have become worthless in this world and the Hereafter, and those are the companions of the Fire, they will abide therein eternally. (217)
It was reported that Ibn ‘Abbaas, may Allah be pleased with father and son, said that this phrase (so do not wrong yourselves therein…) referred to all the months, then these four were singled out and made sacred, so that sin in these months is more serious and good deeds bring a greater reward.
Qataadah, may Allah be pleased with him, said concerning this phrase (so do not wrong yourselves therein…) that wrongdoing during the sacred months is more serious and more sinful that wrongdoing at any other time. Wrongdoing at any time is a serious matter, but Allah gives more weight to whichever of His commands He will. Allah has chosen certain ones of His creation. He has chosen from among the angels Messengers and from among mankind Messengers. He chose from among speech the remembrance of Him (dhikr). He chose from among the earth the mosques, from among the months Ramadan and the sacred months, from among the days Friday and from among the nights Laylat al-Qadr, so venerate that which Allaah has told us to venerate. People of understanding and wisdom venerate the things that Allaah has told us to venerate. (Summarized from the Tafsir of Ibn Kathir, may Allah have mercy on him. Tafsir of Surah at-Tawbah, ayah 36).
May Allah save us from disobedience and increase our worship. Ameen. Please include us in your du'as. Jazakallahu khairan.
And Allah knows best.