Do you use 'a' or 'an' before the word historical?

"a" historical would be correct. ("a" is an indefinite article)
So, "a" before a consonant, and "an" before a vowel.
The exception is for silent consonants, such as "hour".
So, instead of "a" hour, that becomes "an" hour, purely because, in this case, the "h" in hour is silent.
Hope this helps,
John

There is a lesson on an or a here.

It's not really about whether the first letter is a vowel or a consonant. It's all about the sound of the first letter. Does it sound like a vowel or a consonant.

An hour
A house

A united team
An unidentified team

The issue with historical is that some pronounce the h and some don't. Here's the current guidance as we see it:


When pronouncing the words historic and historical, the accent falls on the second syllable, and many pronounce them as starting with a vowel. For those people, it is appropriate to use an before historic and historical. Therefore, you have a choice depending on what sounds best for you.

There is a lot of leniency on this issue. If you're still unsure, opt for a historical and a historic as these remain preferable – especially in formal writing.