English time prepositions are fixed by unit: 'at' for clock times, 'on' for days, 'in' for months and years. So which fits 'I'll meet you ___ Monday morning'?

English fixes time prepositions by the type of time unit — "at" for precise clock times, "on" for specific days or dates, and "in" for longer spans like months, years, or seasons — and a specific day's morning still counts as part of that day, so it keeps "on": "on Monday morning."

"In" would apply to something like "in the morning" generally, without naming a specific day, and "at" is reserved for exact clock times like "at 9am," so neither fits once a specific day is named alongside "morning."

This system trips up many learners because "in the morning" alone uses "in," while "on Monday morning" uses "on" — the day name, not the word "morning," decides which preposition wins.