Yes, because the "it" in the English sentence "It is a warm day" is a dummy subject. It doesn't actually mean anything, it's just there to follow the grammar rules. What the sentence really means is "The day is warm".
We don't use dummy subjects in Esperanto, so we just say "Estas varma tago".
Only use "ĝi" where you could replace it with a noun. The 'it' must be a real thing you can give a name to. "Ĝi estas bela" (It is beautiful - meaning a specific thing, like a vase, or a cat, or a piece of music).
The "it" in English doesn't actually refer to anything; it's a placeholder. You can't replace "it" with the proper noun without changing some aspect of meaning, so Esperanto doesn't use that subject-placeholder. You could say "Ĉi-tago estas varma tago" but that is awkward and clunky, so that's why there's no subject in those kind of sentences. You could only use "ĝi" if you were referring to a specific day. "La 1a de Julio estas morgaŭ" "Jes, ĝi estos varma tago."