Here's why:
* Appositives are phrases or words that rename or explain a noun or pronoun.
* In this sentence, "Day" is the noun being explained.
* The phrase "February 14 birthday" tells us more about the day, specifically that it is Valentine's Day.
However, the sentence is grammatically incorrect. It needs an article ("the") before "Day" to make it a complete sentence. A more accurate sentence would be:
* "The Day, February 14th, is a birthday."