Great question! "It was morning," "It was Sunday morning," and "On Sunday morning" are commonly used in both spoken and written English. "It was a morning" is typically used to tell stories, especially in books, to create a scene or set a mood. For example: "He called me on Sunday morning." → More informal, commonly used in everyday English. "It was *a* cold Sunday morning, and the world outside was covered in a blanket of snow." → More descriptive and artistic, often used to tell a story. I hope this helps!

These can all be fine. Which one you use depends on context: It was morning when I heard the sound of thunder. (This emphasizes the time of day.) It was a morning in April when I heard the sound of thunder. (The speaker may not remember which morning, only that it was in April.) It was Sunday morning when I heard the sound of thunder. (This emphasizes the day of the week.) It was a Sunday morning in April when I heard the sound of thunder. (The speaker remembers it was a Sunday in April, but not which Sunday, ie, first? second? third? fourth?). On Sunday morning we're going out for brunch. (This implies you plan to go to a restaurant to eat brunch the very next Sunday.) We're going to meet for brunch on a Sunday morning next month. (You plan to meet to eat brunch on a Sunday morning the following month, but you have not yet determined WHICH Sunday, ie, first? second? third? fourth?).