The Cow - 2:217
"They ask you about fighting in the Holy Month. Say: "Fighting on it is a great sin. But to turn away from the way of Allah and to be ungrateful to Him and to the Holy Mosque and to drive out its people from it is a greater sin with Allah. For oppression and cruelty is a greater crime than killing."If they could, they would fight you until they turn you back from your religion. Whoever of you turns away from his religion and dies as an unbeliever, all that he did will be in vain in this world and in the Hereafter. They are the people of the Fire and will abide therein forever."
How many holy months are there?
The Repentance/9:36
The number of months in Allah´s Book on the day He created the heavens and the earth is twelve according to Allah. Four of them are sacred. This is the perfect religion; do not wrong yourselves in those months (by fighting). But if the idolaters fight you in a total war, then fight them in a total war. Know that Allah is with the righteous.