I am not a native speaker of English. My mother tongue is Japanese, and we have no Japanese word class that functions like the English definite article "the."
Therefore, the indefinite article is one of the most difficult word classes to us.
Please see the following examples.
1. What are you going to do before/after breakfast/lunch/dinner/work/school/etc? (No need to say, "before your breakfast," "after your breakfast," etc.)
2. I went out before/on/after receipt of his letter. (No need to say, "before my receipt of his letter," "on my receipt of his letter," etc.)
3. You need to pay the charges before/at the time of/on/after delivery of the product (No need to say, "before the delivery of the product," etc.)
May I understand that if an abstract noun comes after words or phrases expressing a moment or period of time (e.g., before, after, on, at the time of), the definite article "the" is not required before the abstract noun?
Your observation is partly correct. After/before lunch, before/after/on receipt of his letter, before/at the time of delivery are all prepositional phrases acting as adverb and so definite article or possessive adjective is not required before the noun. However when the prepositional phrase acts as adjective, article is necessary. As:
I'm after a better chance/opportunity
He was a man after the hopes and expectations of his father.