Hi Sharon!
Airbnb considers 28 days as a month. I believe it has to do with most states consider a guest a long term tenant after 30day stays or more (lots of other issues with this for a host). Below is the Help Article on how monthly discounts are applied:
Weekly and Monthly Discounts
https://www.airbnb.com/help/article/1233
"Monthly discounts apply to reservations of 28 or more nights. The discounted price will be highlighted in the price breakdown on your listing and next to the original in search results if it exceeds 10%."
Some Cautions:
Long Term Tenant Rights
I would be sure you carefully review your state guidelines regarding long-term tenants and their rights. Once a guest enters what the state considers a "long-term tenant occupancy", they have many more rights than a short term tenant. If a guest will not leave when their reservation is up, most states require the owner to go through the court system to evict them.
Monthly Pricing, Cancellations and Guest Alteration Requests
I would also carefully review any long-term stay reservations and ensure the calculations are correct. In addition, the monthly stay cancellation policy is different from your standard cancellation policy. Guests can also request alterations of their stay that different from shorter stays.
Guest Reservation Alterations - Monthly Stays
https://www.airbnb.com/help/article/1361
Hope this helps!