Let's break it down:
* Cual: This is the interrogative pronoun "which" or "what."
* fue: This is the past tense of "ser" (to be).
* la: This is the definite article "the."
* fecha: This means "date."
* de: This is the preposition "of."
* ayer: This means "yesterday."
So the entire phrase asks about the specific date of the day before.