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Which is correct?
(1) Before a holiday, we make plans to do a lot of things on that day.
(2) Before a holiday, we make plans to do a lot of things during that day.
(3) Before a holiday, we make plans to do a lot of things in that day.
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#1 is most common.
#2 is correct.
#3 is not idiomatic.

We'd say, "Try to finish your work during the daytime." "I work better in the daytime."

We would not say, "I'm planning to do that on the daytime.

(I added this to show how we might use "in." We're speaking of "daytime" as opposed to "nighttime.")
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Hi,
Particularly in speech, it's common to use no preposition at all.
Before a holiday, we make plans to do a lot of things that day.

'That day' is commonly understood as implied, so we'd often just say
Before a holiday, we make plans to do a lot of things.

Best wishes, Clive
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Comments  
Hi
Are both of these sentences correct?

I'm driving home on Sunday night.
I'm driving home Sunday night.

Which is grammatically more correct?
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Are both of these sentences correct? Yes

I'm driving home on Sunday night.
I'm driving home Sunday night.

Which is grammatically more correct? #2 is more standard.

Clive

PS - Oops! I meant to write that #1 is more standard..