"In the morning, in the evening, ain't we got fun...." (Words to an old song for all you young'uns.)
For me it works best to excercise of a morning. I like to sit out on the deck of an evening. In the summertime before we had air conditioning, sometimes of an evening we would take a ride in the car to cool off. I like to read of an afternoon.
Budbo makes fun of me if I say I do something "of a morning" or "of an evening." He says I should say "in the morning" etc. Most of the time I do say it that way. I know I speak the way I do because Mom would say "of an evening" and so did her Mother.
The first time I tried looking it up on line, I found a website relating it to people of Irish and Scottish descent. I can't find that website now. But www.thefreedictionary.com states: "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".
It also says it is informally used to indicate a day or part of a period of time when some activity habitually occurs. (Which is generally the way I use it.)
The most southern of my ancestors that I know of were the Duncans. Great, great grandpa Duncan was born in Virginia, eventually coming to Iowa via Michigan and Wisconsin.
Do you sometimes say "of an evening"? Do you know why? This vernacular form of usage may be fading out, but I kinda' like it. Perhaps one or more of my kids will continue saying they do something "of an evening."