Translation:Sunday mornings, dad buys us croissants.
This sounds wrong to me. I'd say "On Sunday morning(s), dad buys us croissants."
Is this natural without "on" to any other native English speakers?
Why is this Sunday mornings? Why not just on Sunday morning? And how would you specific one Sunday morning?
Using the definite article "le" for a day of the week means every occurrence of that day.
- le dimanche = every Sunday
- le lundi = every Monday
To specify a particular day, you have several choices
- lundi, je vais à Paris = on Monday, I'm going to Paris
- ce lundi = this Monday
- ce lundi-là, I went to Paris = that Monday, je suis allé à Paris
So you can leave off the article when specifying a particular day (or at least if you phrase it as you did)? Could you say "sur lundi"? Or is sur more physically on something?