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."?

Question

Answer

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".

Answer

It's a holiday in the US.

Answer

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.

Answer

Not "an" with "US", nor with "United States".
It's a US holiday. - ok
It's a holiday in the US. - ok

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