Surah 9:36
"Surely the number of months with God is twelve, (written) in the Book of God on the day when He created the heavens and the earth. Of them, four are sacred. That is the right religion. Do not do yourselves evil during them, but fight (against) the idolaters all together, as they fight you all together, and know that God is with the ones who guard (themselves)."
This verse commands the “believers” (vv. 34, 38) to fight the enemy, here identified as the idolaters ( mushrikūn ). The verse seems to take away any limits to the fighting in terms of “sacred” months and to envision a reciprocal response to the fighting of the enemy.
- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam
Here also is a brief explanation of the “sacred months” referred to in verse 5.
- from The Quran with Christian Commentary: A Guide to Understanding the Scripture of Islam