Usually, 'the' is used when talking about something specific. Not in these cases
I watched TV in the evening.
means evening in general, not some specific evening necessarily.
We had a barbecue on Sunday evening.
means a specific evening, not any evening. It's some basic, first-grade stuff. And yet, I still can't fully grasp it because it makes no sense to me. Please explain without resorting to lame things like 'it's a phrase', 'it's a convention', 'it's just how we speak', etc.