Can I say "It's an US holiday" or should I say "It's a holiday in the USA."
It's a national holiday in the USA next Monday.
Can I say "Next Monday is an US holiday."?
In this case, you can say
It's a national holiday in the USA next Monday.
or
Next Monday is a US holiday.
You need to use "a" in the second sentence because the "U" in "US" is pronounced like "you".
2019년 8월 27일
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It's a holiday in the US.
2019년 8월 27일
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The words in your question are even better.
(Next) Monday is a national holiday. (If you are in the US)
(Next) Monday is a national holiday in the US. (If you aren’t)
Next isn’t necessary either.
2019년 8월 27일
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Not "an" with "US", nor with "United States".
It's a US holiday. - ok
It's a holiday in the US. - ok