Hello @Daniele941 @Breanna
To be able to create a specific promotion like this you will have to enable "professional tools" that despite some of the wordage on some of the links on this subject is now available to all hosts but only from a computer.
So once you've opted in (it is reversible) you will see a new linear calendar that Airbnb call the "multi calendar". I'm including the great Guide that @Joan2709 has compiled and illustrated so that you have a better way of understanding what the pro tools offer.
https://community.withairbnb.com/t5/Community-Guides/GUIDE-Use-Pro-Tools-to-Set-Calendar-Custom-Pric...
It might be an idea to click on the above before reading further because it might get a bit confusing!
So my idea for you to make a promotion for every Monday and Tuesday would be to "create a rule set" first. Give this rule a name ie. Monday/Tuesday promotion and attribute one of the colours to select from to this new rule set.
==>Enter a nightly price for these two days and now you have created a rule.
Next go to the multi calendar and highlight every Monday and Tuesday on the calendar and attribute the coloured rule set you have just created to these specific dates on the calendar. Always double check in guest mode that this has worked but with the calendar in host mode will show that a rule set is applied to every Monday and Tuesday because it has a specific colour so easily identifiable.
You never know, someone might come back to you with another idea but you do have quite a specific demand that doesn't fall under Airbnb's usual promotion possibilities😉
Let us know how you get on,
All the best
Joëlle