of an

alternative of a

1. datedHaving the same.

   The two main players were roughly of an age.

   3 more examples

   He and F were of an age and standing, the giants of the house, but F was the braver, and did us the good office of keeping him in order.

   1825 March 16, Robert Southey, “Letter XVII”, in Life and Correspondence of Robert Southey, page 56

   Allen said, "If we were of an age, and on an equal footing, you would not give me the lie so often."

   1854, Elias Darnell, Journal of the Campaign, page 74

   When she turned to kiss Alcuin, they were of a height.

   2001, Jacqueline Carey, Kushiel's Dart, →ISBN, page 73

2. datedidiomaticIndicates a more or less habitual activity during the given part of the day.

   Of an evening, I like to play chess. i.e., On some evenings, I like to play chess.

   1 more example

   Of a morning, they would work in their garden. i.e., They generally worked in their garden in the morning.