What indefinite article (a or an) should be used before "historical"?
an historical standpoint.
an historical misunderstanding.
The answer is "a historical ..." because the "h" in front of "historical" is pronounced, and therefore "historical" doesn't begin with a spoken vowel, and "an" is to be used when the word begins with a spoken vowel, like "hour" (pronounced "our").
The simple rule is, "Use 'a' before a word that begins with a consonant sound. Use 'an' before a word that begins with a vowel sound."
As Stangdon notes in his comment on another answer, the key is the SOUND and not necessarily the spelling.
"Historic" is a classic ambiguous case because some dialects pronounce the "h" ("h" as in "here") and others don't. Those who do pronounce the "h" often pronounce it only slightly. So there's either no consonant sound or only a weak consonant sound. Thus "an" would be appropriate. But if you pronounce the "h" clearly, then "a" is appropriate.
If you are simply following the rules of English, you use "a" in front of consonant sounds and "an" in front of vowel sounds. Virtually all modern English dialects begin "historical" with a consonant sound, so "a historical" is correct for the modern pronunciation in most dialects. If you speak a (somewhat uncommon) dialect of English that features phonetic H-dropping for the word "historical," then you might use "an historical".
However, there is a tradition (mainly in British English) of using "an historical" in modern text, despite the fact that "an 'istorical" is not generally spoken aloud. This is possibly due to adherence to the traditional form of "an historical": at some point in the past, the pronunciation "an 'istorical" was more common, and certain British authors chose to persist this formulation in text long after the silent-H pronunciation had dropped out of common use. Alternatively, John Lawler suggests this is simply a matter of syllable stress so that some dialects still soften the H in "historical" to merit use of "an". I speculate that uncovering the reason and history of this persistence is potentially a collegiate-level research topic.
If in doubt, "a historical" is safest. If you wish to persist the British affectation of "an historical," you may do so at the risk of being corrected by people following the standard rules of English based on modern pronunciation. In any case, "a historical" is more popular than "an historical" and has been since around 1940.
The rules for using "an" instead of "a" is really only about what sounds better. You see both used because of accents. Some British accents don't pronounce the H at the beginning of "historical". So saying "a istorical" is difficult, so "an" is used.
In most North American accents, the H is pronounced. So it's easier to say "a historical".
Which you use when writing will depend solely on who your intended audience is.
"An historic" sounds more natural to me than "a historic," even though I do pronounce the H. The same goes for "hilarious," "heroic," and "habitual." I think it's because the first syllable is unaccented.
In the US the use of "an historical" is becoming more and more common among American elites and aspiring elites. It was almost unheard of twenty years ago. It is being used as a sign of class distinction In our increasingly stratified society. Those who use it are making deliberate statement that they are not of "The Great Unwashed Masses." I lived among elites throughout the 1980's. They never used "an." I worked at an elite Northeastern college from 2004 to 2014. In 2004 professors and high ranking administrators almost universally used "a." By 2012 at the latest those very same administrators were using "an." The professors' usage was mixed at the time I left. I took note of the affectation several years ago and continued paying attention to it over the years because, at first, I thought it was odd. Over time it became clear to me what was going on. Another good example of this phenomenon is National Public Radio. Their commentators never said "an" before "historical" until about 8 years ago. All of their commentators have been using it for the last couple of years. Some of their reporters still use "a." But, I am sure they have to change their ways to get a promotion.