Zul Hijjah is from the four sacred months of the Islamic calendar. The four sacred months are Zul Qa’dah, Zul Hijjah, Muharram and Rajab. ^[1] The rewards of good deeds carried out in these months are multiplied, and the sins committed in these months are also regarded as more serious. ^[2] Allah Ta‘ala says:

إِنَّ عِدَّةَ الشُّهُوْرِ عِنْدَ اللهِ اثْنَا عَشَرَ شَهْرًا فِيْ كِتَابِ اللهِ يَوْمَ خَلَقَ السَّمٰوٰتِ وَالْأَرْضَ مِنْهَا أَرْبَعَةٌ حُرُمٌ

The number of months in the sight of Allah Ta‘ala is twelve (in a year), so ordained by Him the day He created the heavens and the earth; from among them four are sacred. ^[3]