"É um dia histórico."
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".
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."