"It is a historic day."
Translation:Es un día histórico.
The rule "a = feminine / o = masculine" has lots of exceptions and yes, this is one.
A good way I have found is if you're confused as to whether it's el or la, I ask myself, which sounds the least weird...for example: "La agua" sounds weirder than "el agua". "La dia" sounds weirder (like ladia almost) than "el dia". Spanish is all about things sounding nice! :D
For me the reminder that "día" is masculine is the Spanish version of the Jingle Bells song where in the refrain they sing "Navidad, Navidad, hoy es navidad, Es un día de alegría y felicidad". Go to yotube, listen to the song three times and you will never forget that you should say "el día", "un día", "un día historico" etc.
Words that end in 'a' but are masculine are normally old greek words e.g. el planeta, el problema, el agua, el dia - such words are normally very similar to their English counterpart.
"An," is used when the letter immediately following it is not sounded. Unless you pronounce historic as 'istoric (as some do), "a" is used. Same with, "an 'erb," (US) and, "a herb" (UK).
If the word following "a" is vowel sounding, then it turns from a -> an.
This is a traditional way of saying that sentence in English but it is not commonly used anymore outside certain newspapers. Though technically if you translated it to say an then you would be correct.