"Was the party yesterday boring?"
This sentence can also be translated into English as "Was yesterday's party boring?" The party belongs to yesterday, so in this case using the の is normal, just as in English we use 's.
If the sentence is changed a little by adding "went" you could drop the の.
Was the party you went to yesterday boring?
It would work because if I were to say yesterday's party, it means the same thing. Alec's party yesterday would require 2 particles.