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{Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him]} [at-Tawbah 9:36].
The scholars said: There are some benefits in making some months more sacred than others, such as the fact that there is an interest to be served when people refrain from committing transgressions and evil deeds in the sacred months, because Allah gave them a high status, for perhaps that might lead to giving up wrongdoing completely, as the motive for such actions may diminish during that period. End quote.
https://islamqa.info/en/answers/317278/why-is-rajab-separate-from-other-sacred-months