Fighting during the Sacred Months is prohibited, except in Self-Defense

Ibn `Abbas, Ad-Dahhak, As-Suddi, Qatadah, Miqsam, Ar-Rabi` bin Anas and `Ata said, "Allah's Messenger ﷺ went for `Umrah on the sixth year of Hijrah. Then, the idolators prevented him from entering the Sacred House (the Ka`bah in Makkah) along with the Muslims who came with him. This incident occurred during the sacred month of Dhul-Qa`dah. The idolators agreed to allow them to enter the House the next year. Hence, the Prophet entered the House the following year, along with the Muslims who accompanied him, and Allah permitted him to avenge the idolators' treatment of him, when He said:

الشَّهْرُ الْحَرَامُ بِالشَّهْرِ الْحَرَامِ وَالْحُرُمَـتُ قِصَاصٌ

Ruling

There are four 'sacred months': Dhul-Qa'dah, Dhul-Hijjah, Muharram which are consecutive and the fourth is Rajab. Even before Islam, fighting in these months was considered unlawful and the disbelievers of Makkah too used to observe their sanctity. Even in the early days of Islam, right through the year 7 A.H., this law was in force which is why the noble Companions were perplexed. Later, according to the consensus of the Ummah, this unlawfulness of fighting was abrogated and the permission to fight was given, but it is still preferable not to initiate fighting in these four months without the need to defend. Therefore, it can be said that the sanctity of the 'sacred months' has not been abrogated totally, but it holds good like the sanctity of Haram. Both of them have been subjected to an exception for the sake of defence.