Of an evening

[LittleRedDinosaur]

[ICameOnTheJitney]

What about "On an evening" or "On a morning"

[ohmymimi]

They both sound ok to me, as I grew up hearing them used frequently (in a village in Worcestershire). Don't they qualify as vernacular usage?

[TallulahBetty]

I say "of an evening". I know it's not grammatically correct, but it's local dialect. I say it but never write it.