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Use ‘an’ rather than ‘a’ if it is followed by a vowel sound (i.e. not necessarily a written vowel):
an hour, an upset, an incident, a youth, an impossible coincidence, an evident joy (I’ve just noticed that this sounds like a minimalist short story)
The convention for writing ‘an historic’ arose towards the end of the nineteenth century, when it briefly became fashionable for the upper classes to drop their aitches. I don’t know why the habit retained its grip when the fashion passed. Some Americans say ‘erbs, rather than herbs, for the same reason.
There’s no need to keep this convention going, so write ‘a history’ or ‘a historic occasion’ unless you think it will offend your readers.
With acronyms and abbreviations, you should still obey the rule to use ‘an’ if it is followed by a vowel sound, even if this would be wrong when the abbreviation is expanded:
an EU treaty (this is right, even though you would write ‘a European Union treaty’)
a US election (this is right, even though it is followed by a vowel – go by what you would say)
an SFO investigation (this is right, even though it is followed by a written consonant, it is still a vowel sound: ‘ess-eff-oh’)
a SERC investigation (this is right, despite the above, because you would say ‘serc’ rather than ‘ess-ee-ar-see’)