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Which is correct out of the following & why?

I will be back on Monday.

I will be back by Monday.

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Both are correct but they mean different things.

To be back by Monday means not later than Monday but it could be Saturday or Sunday or any other day previous to Monday.

To be back on Monday means on that day and not before or after.

As to why it's because that's what the prepositions signify.

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As far as I know, both of them are correct. However, there is a slight difference in meaning and usage.

In this sentence:

I will be back on Monday.

you mean you will come back exactly on Monday, not before.

In this sentence:

I will be back by Monday.

you mean that you will come back on or before Monday.

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Both are correct. "I will be back on Monday" means "Monday is the day I will come back"; "I will be back by Monday" means "I will come back no later than Monday".

On

8 Indicating the day or part of a day during which an event takes place.

‘reported on September 26’
‘on a very hot evening in July’

On (Oxford Dictionaries)

By

4 Indicating a deadline or the end of a particular time period.

‘I've got to do this report by Monday’
‘by now Kelly needed extensive physiotherapy’

By (Oxford Dictionaries)

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