Wouldn't the word unique be an exception to the rule?
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With 'historic', it depends on how you pronounce it. Brits would say "A historic day" with a hard H. I think Americans would say "An historic day" with a silent H.
In the latter case, both options ("a" and "an") are often available.
a unique, a uniform, a university, a universal problem, a unilateral agreement, a united nation, a once-in-a-lfetime opportunity
an hour, an honest man, an heir
Concerning the word historical, here in Canada most people would go for A Historical as well as in the US. However, I do hear people say AN Historical here in Canada and I think I myself say it like this sometimes, but in this case the letter H is not pronounced. But before HISTORY, always A.