That is a tag question. A tag question follows a statement, positive or negative and I use it when I think you know the answer, but I want to be sure. So, I think you know that the Lunar New Year is for 15 days. I don't ask, "How long is the Lunar New Year?" I begin with a statement = The Lunar New Year is 15 days and then I follow my positive present tense statement with a tag question: Isn't it?
Can you explain this kind of question: Lunar New Year lasts for 15 days in March, doesn’t it?