on Saturday morning — day + part of day (GiU Units 119-121)
The frame
Section titled “The frame”I’m going away on Saturday morning. ✓
The day governs the phrase: any part-of-day attached to a named day keeps on.
The three-way time split
Section titled “The three-way time split”| preposition | used with | examples |
|---|---|---|
| at | clock times · night · the weekend (BrE) · holiday periods | at 8 o’clock · at night · at the weekend · at Christmas |
| on | days and dates | on Monday · on 12 September · on Saturday morning · on my birthday |
| in | months, years, seasons, parts of day (no day named) | in July · in 2026 · in summer · in the morning · in the evening |
Rule: parts of day take in alone (in the morning) but on once a day is attached (on Saturday morning). Exception: at night (never “in the night” for the general sense).
Detail worth knowing
Section titled “Detail worth knowing”- at night vs in the night — at night = generally during nights; in the night = during one specific night (“I woke up in the night”)
- on the weekend (AmE) vs at the weekend (BrE)
- No preposition: next Monday, last July, this morning, tomorrow
- on for dates needs an ordinal feel: on 5 May (BrE) / on May 5th (AmE)
Source
Section titled “Source”English Grammar in Use, Additional Exercises — Prepositions (Units 12, 119–136). Ebot lesson 2026-09-05.