"É um dia histórico."
Actually it should be "an historic" because the stress is on the second syllable and in such word beginning with "h" the indefinite article should be "an" and not "a". However I did not dare write this for fear of loosing a heart. But it should be accepted as correct.
I thought the rule always was whether the "h" was silent or not. In the case of "historic" the "h" is pronounced, and hence "a".
Barbeito is correct. Here's a quick and dirty link that explains when to use 'a' vs 'an' and lists some of the exceptions: https://owl.english.purdue.edu/owl/resource/591/1/
Just as srshti says it depends on how you choose to pronounce the words (by dropping the 'h' or not). There is probably no real right or wrong though, at least for words like "historic". See http://oxforddictionaries.com/words/a-historic-event-or-an-historic-event for a discussion tuned to this particular word (they don't agree with Barbeito). If you think that link has a British bias, never fear, the US Merriam-Webster Dictionary doesn't agree either: http://www.merriam-webster.com/dictionary/historic.
What probably fooled you is that the only 'h' words discussed in your link do not sound the 'h' and therefore take "an".
Hmm. This serves to remind me how accomplished I am at making mistakes in my own language. :)
Both are perfectly acceptable in English. In other words, neither is wrong :)
I'm a little bit confused. I thought it would be "historical day". Could anyone explain, when to use historical and when historic?
From the Oxford Dictionary: "Historic and historical are used in slightly different ways. Historic means ‘famous or important in history’, as in a historic occasion, whereas historical means ‘concerning history or historical events’, as in historical evidence: thus a historic event is one that was very important, whereas a historical event is something that happened in the past."