Translation:

36. The number of months in the sight of Allah is twelve (in a year) so ordained by Him the day He created the heavens and the earth; of them four are sacred; that is the straight usage. So wrong not yourselves therein and fight the pagans all together as they fight you all together. But know that Allah is with those who restrain themselves.

Notes (Tafseer)

1295. This and the following verse must be read together. They condemn the arbitrary and selfish conduct of the Pagan Arabs, who, because there was a long-established custom of observing four months as those in which fighting was forbidden, changed the months about or added or deducted months when it suited them, to get an unfair advantage over the enemy. The four Prohibited Months were: Zul-qa'dah, Zul- hijjah, Muharram, and Rajab. If it suited them they postponed one of these months, and so a prohibited month became an ordinary month: while their opponents might hesitate to fight, they got an undue advantage. It also upset the security of the Month of Pilgrimage. This very ancient usage made for fair dealing all round, and its infraction by the Pagans is condemned. The question of a solar astronomical year as against the lunar Islamic year does not arise here. But it may be noted that the Arab year was roughly luni solar like the Hindu year, the months being lunar and the intercalation of a month every three years brought the year nearly but not accurately up to the solar reckoning. From the year of the Farewell Pilgrimage (A.H.10) the Islamic year was definitely fixed as a purely lunar year of roughly 354 days, the months being calculated by the actual appearance of the moon. After that, every month of the Islamic year came about 11 days earlier in the solar year, and thus the Islamic months travelled all round the seasons and the solar year.

1296. The Muslims were at a disadvantage on account of their scruples about the Prohibited Months. They are told not to wrong themselves in this. If the Pagans fought in all months on one pretence or another, they were allowed to defend themselves in all months. But self-restraint was (as always) recommended as far as possible.