In a day or on a day? What’s the difference? And what about on the day?

In a day means in the duration of a day (24 hours). The expression is used when an event or action happens within the duration of a day. Here’s an example:

  • Suzy finished the project in a day.

In the example above, the subject of the sentence, Suzy, complete her project within the duration of one day. In other words, she complete her project within 24 hours.

On a day means on such a day. The phrase is used to convey that the day in question is a day that is different from a typical day. It’s a day that has a quality that sets it apart from other days.

  • I can’t believe Nate is going to propose to Zoe on a day like this!

In the example above, Nate is going to ask his partner to marry him on a day that is not suitable for a proposal. This day in question is different than other days that would be socially acceptable for a proposal.

On the day means on a certain, specific day. It is used when denoting a particular day.

  • James resigned on the day of the company-wide pay raise.

Here, we can see that James quit his job on the specific day that the company issued a pay raise for all its employees. What timing!

We’ve prepared some practice questions you can try out to augment your understanding of the three phrases. You can find the answers at the bottom of this article.

  1. She gave birth in a day/on a day/on the day of her wedding anniversary.
  2. Wayne swiftly built a tree house in his backyard in a day/on a day/on the day.
  3. Why is Paola trying to wash her car in a day/on a day/on the day when it’s raining this much?
  4. Our manager wanted the marketing proposal to be finished in a day/on a day/on the day so it could be presented the very next day.
  5. Justine ate the whole cake by herself in a day/on a day/on the day.
  6. I’m glad I started my first job in a day/on a day/on the day like this; it’s perfectly sunny with not a single cloud in the sky.
  7. Grace did not want to go to the amusement park in a day/on a day/on the day of her father’s death anniversary.
  8. Ulysses was excited he got a bonus in a day/on a day/on the day before his daughter’s birthday.
  9. Why would you do a picnic in the park in a day/on a day/on the day this hot?
  10. Did you finish the summer assignment all in a day/on a day/on the day?

If you still are unsure about which to use, try out where you can submit your English sentences to get immediate feedback and suggestions based on how native English speakers write.

Answers to the practice questions:

  1. on the day
  2. in a day
  3. on a day
  4. in a day
  5. in a day
  6. on a day
  7. on the day
  8. on the day
  9. on a day
  10. in a day


What is the difference between “in the day”, “on the day” and “during the day” in context?
Tell me please the difference between the following sentences. February 11 was really hectic, so I had to do a ton on the day. February 11 was really hectic, so I had to do a ton in the day...
What is the difference between “IN a day ” and “ON a day ” ? “IN a day ” vs “ON a day ” ?
Synonym for IN a day “in a day” is usually referring to how much time was needed to do something. For instance, “I can paint that house in a day.” means that it will only take one day to do it. “On a day” is used to specify a date. For instance, “I can meet with you ON Thursday.” “He will return on…