In the song, Last Christmas, I heard the phrase "But the very next day." I'm not sure what it was supposed to mean, but from context I guess it's the day after Christmas
Is it grammatically correct to say "very next"? Something is next or is not next. Can something be 'more next' that something else? Can something be 'very next' or just 'a little next'?
It is grammatically correct to say "very next". Very next day means the day after a certain event happened or happens. It means same as the next day but with an emphasis (to denote the short time period) and is used only in time-sensitive contexts and not every time one wants to refer the next day. For ex -
John was not able to go to school that day but the very next day he recovered and went to school all fine and dandy.
Instead of returning to London three days later, as he said he would, he came back the very next day.
Very is used for emphasis, not just to mean in an high degree (which is the meaning it has in very big). It is also used in phrases like the very best quality, her very own apartment, and the very beginning of the book.
You are correct in assuming that you can't be more next than something else.
I'm assuming you're referring to the song "Last Christmas", and the line
Last Christmas I gave you my heart. But the very next day, you gave it away.
In that situation, adding the "very" is an informal way of stressing how quickly it happened. Also keep in mind it's the following day, so December 26th, not Christmas Eve.
Very here means true and is ultimately derived from Latin verus, "true". ODO definition
It's only used in set phrases such as "very next day", "his very own", "the very beginning" and so on, and emphasises that it was actually the next day; actually his own; right at the beginning.
