Hi Cindy,
I think you should start your pricing considering the most average situation. Which is the 143,- a night ? With min. stay set to 7 nights and max. stay set to 28 nights. For special periods you can setup the higher price, on those days or as weekly rates for specific weeks. If this is only for a few holiday periods, i think setting the higher price per night "hardcoded" on those limited days in the calender will do the job. I think (but not sure!) your listing will then show initially potential guests the 143,- price per night. Sometimes you have to experiment.
Maybe you would also like to offer guests a monthly discount if the book 4 weeks ? Or maybe you want them check-in only on certain days ? It is all possible.
But allways simulate a booking on your own listing, to check what a guest is offered is right, by simple using the example of your listing, or goto: https://www.airbnb.com/rooms/21750128 . If it's not showing the prices which you expect, do some more tuning until your a satisfied.
Hope this helps,
Emiel