"Сегодня холодный день."
Translation:Today is a cold day.
Your choice translates as "сегодня холодно". It has the same meaning, but doesn't have a word "day" = "день".
Exactly right: сегодня is the subject, день is the object. It is imperative that both of these words are present in the translation: today, and day. You cannot switch one for a pronoun. That would be equivalent to saying that "Анна ест яблоки" could be translated as "she eats apples".