Every year, God wanted the children of Israel to observe the Day of Atonement,

“This shall be a statute forever for you: In the seventh month, on the tenth day of the month, you shall afflict your souls, and do no work at all, whether a native of your own country or a stranger who dwells among you” (Leviticus 16.29).

God chose the seventh month for a reason. He could certainly be arbitrary if He so desires, but the Atonement of the tabernacle and of the people ranked high on His list of priorities.

Therefore, He wanted the Day of Atonement observed during a month that was important to Him.