Learn Russian: Почему вам было холодно вчера?

In Russian, physical states and feelings (like being cold, hot, or sad) are often expressed as impersonal sentences. Instead of saying "you were cold," Russians say "it was cold to you." This is why we use an adverb like холодно instead of an adjective.

Why do we use вам here instead of вы?

Because of the impersonal structure, the person experiencing the feeling is not the grammatical subject. Instead, they are the receiver of the feeling, so we use the Dative case. Вам means "to you," whereas вы is the Nominative (subject) form.

Why is the verb было in the neuter form if it is talking about a person?

In impersonal sentences, there is no active subject (no "he", "she", or "it" doing the action). Without a subject to match genders with, the Russian past tense always defaults to the neuter singular form: было.

Does вчера have to be at the very end of the sentence?

No, Russian word order is quite flexible. You could easily say Почему вчера вам было холодно? Putting вчера at the end just places a slight emphasis on the time.