To light up is not the phrase for this phenomenon. To brighten, to get lighter would be better.
To light up requires a direct object, e.g. ”to light up the room” as in ”to make the room get bright”; ”to light up a cigarette”.
That would not be an accurate translation, as the verb is in infinitive. The correct translation of this expression using your idea would be "To brighten up". I know, it's not making sense and it's useless in conversation, but it is what it is.
why do you change it? "A se lumina" (To brighten up-infinitive) is not the same as "S-a luminat de ziua" (It brightened up-past tense). In order to use a verb as a predicate in everyday speech, it shouldn't be in infinitive form, but rather in the finite form.