Do you say this when you want to refer to a date which is exactly 366 days ago?
Can you use the same form for 367, 368, etc.
All of the above. :-) Just as in English πέρυσι can be used to mean various dates.
I think the question was: "does 'πέρυσι και χτες' mean 'one year and one day ago'?"
i.e. is it a fixed phrase referring to one specific day?
Or is this sentence simply two separate concepts: 'πέρυσι' and 'χτες'?
I think it is just two words referring to past time. I can't think of any fixed expression in Greek. (In English we have "a year and a day" for prison sentences but that has nothing to do with this sentence. ;) )