@MariaAlejandra0 The simplest way to do it is use the Pricing tab weekly and monthly discount. You can go to listing pricing tab to use Length-of-Stay discount, which has built-in weekly and monthly discount, Last minute discount, and Early bird discount. You may use Add custom length to define your own Length-of-Stay discount.
If just for a period of time, you can create rule sets in Multi-calendar page and apply your rule set to the selected periods.
BTW, the normal Special Offer from Airbnb can only be applied when a guest sends you a booking inquiry.