I'm reading kids' textbook.
The question is "Which preposition should you choose?"
We drank the milk ___ lunch.
Is the blank at, to, in, or on?
The book provides those four options.
Learn English – Which preposition should I choose for “lunch”
prepositions
Best Answer
"At lunch" is an idiom that means "while we were eating lunch." More broadly, it may mean the time immediately before eating or immediately after eating but at a place associated with eating such as a restaurant.
"At" is usually an indicator of place. I suspect (but do not have a citation) that "at lunch" etc. derive from the old fashioned phrase "at table," meaning "sitting at a table while eating a meal."