Note: by "parts of the day" I only mean: morning, afternoon, evening, and night. This question is only relevant to these four.


According to Cambridge dictionary:

We use in with morning, afternoon, evening and night, but we use on when we talk about a specific morning, afternoon, etc., or when we describe the part of the day.

I always work best in the morning. I often get tired in the afternoon. The ship left the harbour on the morning of the ninth of November.
In the evening they used to sit outside and watch the sun going down. It happened on a beautiful summer’s evening.


In the night usually refers to one particular night; at night refers to any night in general:

I was awake in the night, thinking about all the things that have happened.

‘It’s not safe to travel at night,’ the officer said.

But if I just follow this logic of a "specific morning,...etc", a logic not just mentioned by Cambridge Dictionary but on Stack Exchange1 2 as well, then sentences like:

Let's meet in the morning (tomorrow).


He passed away in the morning on March 5. (on the morning of March 5)

Should be written as:

Let's meet on the morning (tomorrow). *

He passed away on the morning on March 5(on the morning of March 5). *

As we aren't going to meet on any random morning, we will meet tomorrow's morning, it isn't indicated explicitly but it's implicit that it's tomorrow's morning.

So, I guessed there must be more powerful rule than this one. I searched and found these sentences:

I didn't do anything on Friday night.

Should I let my vehicle warm up on cold mornings?

The coup came on the morning of the referendum. (The New Yorker)

You have to be at the tourist office at dawn the morning before you want to go. (The New York Times, travel)

Everything clicked on a morning when the Ocean Course played long... (The New York Times)

For each person, antibody levels were measured immediately before their jab, and again on a morning one month later. (The Guardian)

Reading these examples, I concluded that and someone else as well:

To be able to substitute 'on' for 'in', I have to have some grammatical modifier describing my morning, evening, etc.

Let that modifier be a simple noun or adjective (Friday, cold), or some more complex modifier phrase (when the Ocean, of the referendum). And I guessed that:

without a modifier even if it's implicit that the speaker is talking about a specific morning the sentence wouldn't sound natural without a modifier, just like the first examples I gave*.

So far, I thought I had reached the ultimate rule! But I soon found these examples which I believe they break the "ultimate rule" I concluded; they have modifiers, in bold, but they still use "in" for "general time":

11/1 in the morning before the exam. (Standford University)

He'd filled himself up in the morning before ken arrived. (The Guardian, film)

The ship set sail in the early morning on May 15th. (TimR's answer)


For the argument I presented*, I don't believe the "specific/ non specific" is an inclusive enough rule. So, my question is, why didn't the rule I concluded work for the last two examples? Or is it correct but it can be broken sometimes?

  • 1
    "Let's meet tomorrow morning" would be more usual, unless it's already quite late in the day. When speaking of the next day, in the morning implies 'when the night is over'. Commented Apr 7 at 12:48
  • @Astralbee Thanks for suggesting that question but it does not. By the end of my question, I include some examples that kind of break that rule included in the suggested question and in my question as well, I'm asking specifically about these examples.
    – Manar
    Commented Apr 7 at 13:21
  • 1
    Not sure about the first one, but the second seems to refer to a time of day rather than to identify a particular morning. In the morning [and] before Ken arrived rather than on the morning [of the day] before Ken arrived. Commented Apr 7 at 13:50

2 Answers 2


Use in the morning when you wish to refer to the morning as a span of time, even if it happens to be on a specific day:

The ship set sail in the early morning on May 15th.

He'd filled himself up in the morning before ken arrived. (The Guardian, film) [though I have to say I have no clue what that means, absent context, other than to say that the speaker is presenting the morning as a span of time in which something took place]

Use "on the morning of {date}" when you wish to refer to the morning as a point in (calendar/chrono) time.

It was on the morning of April 1st that he first discovered that he had become a giant bug. He might have discovered it at some point in the previous evening but the lights had gone out all over the city and the house was pitch black.

  • Hi, I'm not the one who downvoted. Anyway, what about sentences like this one that I mentioned in my question: Should I let my vehicle warm up on cold mornings? This doesn't sound like a point in the calendar.
    – Manar
    Commented Apr 7 at 20:09
  • 1
    It is certainly not taking those mornings as a span (i.e. a contiguous stretch of time). The reference is to a set of individual mornings that share a common attribute ("cold").
    – TimR
    Commented Apr 7 at 20:30

Use "on" when referring to days of the week, dates, or general time periods.


  • We're having a party on Saturday.
  • The party is on the 15th of April.

Use "in" when referring to broader periods of time or to indicate a more general sense of time rather than a specific point or day.


  • We're having a party in the summer.
  • The party is in April.
  • I woke in the middle of the night.

Remember that we also use "at" to refer to specific points in time, particularly when specifying the exact hour or a particular moment.


  • I woke at 2 am.
  • I already know about the other rules you mentioned. My question is only about "morning, afternoon, evening, and night".
    – Manar
    Commented Apr 7 at 20:04
  • It seems like you haven't really read my question body but just the title. Could you please reread it.
    – Manar
    Commented Apr 7 at 20:11
  • @Manar Maybe re-read my answer. Because I believe I have addressed your issue. Your first example is "in the morning". That's a period of time and not a specific date. Your second example is "on the morning of the ninth of November". That is a specific date. I think my answer tells you which to use for each, based on the specificity.
    – Astralbee
    Commented Apr 7 at 20:33
  • What about TimR's answer "The ship set sail in the early morning on May 15th." and "He'd filled himself up in the morning before ken arrived." and other examples I included like: "on cold mornings" cold mornings doesn't sound like a specific date to me. I think your answer isn't inclusive enough.
    – Manar
    Commented Apr 7 at 21:03
  • 1
    @Manar I'm not commenting on somebody else's answer here. I see you've already got a couple of close votes on your question. Questions here need to be focused. You can't list a ton of examples and expect each and every one to be addressed. Answers should be useful to future readers, not just limited to your example. The principles in my answer should allow you to figure out all of your examples. And I still think that you'll find this already answered on the site. I had a choice of about 5 previous questions when I suggested it was a duplicate.
    – Astralbee
    Commented Apr 7 at 21:07

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