YourDictionary.com (scroll to the bottom of the page) quotes The American Heritage ® Dictionary of the English Language:
"Some speakers of vernacular English varieties, particularly in isolated or mountainous regions of the southern United States, use phrases such as of a night or of an evening in place of Standard English at night or in the evening, as in We'd go hunting of an evening."
(Found by searching the phrase: "of an evening" origin. Note quotes around "of an evening".)
A writer for whom I do some proofreading just sent me this question:[blockquote]Do you have a sense of where the expression "of an evening" or "of an afternoon" comes from? I've used it in the context of a man from the Northwest, an educated biologist, and my editor wants me to change it. I'd really like to push back on this one, but I'm worried she might be right. I can't figure out a way to Google it.[/blockquote]I'm sending back saying it certainly isn't something Bruce made up, that I've heard it too and that it's meaning is "once in a while" or "occasionally". But can anyone describe its ancestry or suggest a way to find out?