Are you looking to provide 5% off for the month of December or are you wanting to give a 5% for a month's booking? Somewhat confused by your post. Unless you are using Smart Pricing, just go into your calendar and select the days you want to reduce the rate. Having a lower rate allows for the guest search to include the lower rate. If you want the whole month of December to be booked, you can edit under availability for a specific period of time there is a different minimum. Under pricing you can offer different period of booking discounts.