Hi @Daniele941
Airbnb Custom Promotions are different than creating a Rule Set to apply a % discount...it's kinda confusing.
Airbnb Custom Promotions
This is great, as you can get the "strike-thru" price that a guest will see on your thumbnail when searching (usually 15-20% is required). However, Airbnb has restrictions on the use of Custom Promotions.
Why Can't I Set Custom Promotions for Certain Dates
https://www.airbnb.com/help/article/2719
Rule Sets to Discount Price
As @Joelle43 said, this is another way of discounting your prices. Guests will not see the strikethrough pricing, but will see the discount when trying to book. You can also create a Rule Set to provide a discount if the guest books a longer stay. If you have a 2nite minimum, you can create a Rule Set to provide a discount if they book 3nites or more. Lots of options with Rule Sets.
The only way to apply a discount to specific day of the week (say Mon/Tues) currently, is to create a Rule Set and apply it manually to those days.