\\\\\\\

- Какой день был вчера?

- Вчера было воскресенье.

\\\\\\\

________________________________________

\\\\\\\

VERB  <= NOUN №1
\\\\\\\

When we need to decide about the form of a verb, we have to check the noun in Nominative case (form №1).

\\\\\\\

Thus, in the question the noun №1 is “день”, он (masculine). Therefore we use the verb in masculine: “был”.

\\\\\\\

In the answer the noun №1 is “воскресенье”, оно (neuter). Therefore we use the verb in neuter: “было”.

\\\\\\\

ATTENTION: “вчера” is not a noun at all. It’s an adverb and it’s neither feminine nor masculine nor neuter and never influences the form of a verb.

\\\\\\\

\\\\\\\