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

Đừng bỏ lỡ cơ hội học ngoại ngữ ngay tại nhà. Hãy xem danh sách các gia sư ngôn ngữ giàu kinh nghiệm của chúng tôi và đăng ký buổi học đầu tiên ngay hôm nay!