Why are coupons always set to expire on the last day of the month?

Is it so people can remember the exp. date better or ...something else?Why are coupons always set to expire on the last day of the month?
It is easier to remember. Some online retailers expire coupons at the end of every quarter. Some expire at the end of every month. Some set coupon expiration date based on sales calendar.Why are coupons always set to expire on the last day of the month?
It's more to coincide with accounting IMO.

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