If it is a title then, An, Apple and Day would all be capitalized.
If it is in a sentence, "An apple a day is healthy.", then only An would be capitalized.
The word "an" should be lowercase in the phrase "an Apple a Day." In English, indefinite articles like "an" are typically written in lowercase unless they begin a sentence. However, it's important to note that the word "Apple" should be capitalized because it is a proper noun. Proper nouns, like the names of specific people, places, or things, are typically capitalized. In this case, "Apple" refers to a brand name and should be capitalized. So, the correct capitalization would be "an Apple a Day."