I am paid twice per month, and in past years our Christmas bonus has been paid on Dec 15, after which we receive an end-of-year payroll on or about Dec 30. I asked my admin today why my end-of-year wire had not hit today (being a small company they usually pay on the day the pay period ends) and he told me that the Dec 15 pay included the end-of-year pay. I was not informed of this in writing or verbally until today, and in past years things were not done like this. Am I entitled to pay for Dec 15-today?

You GOT your Dec 15 pay, right? You have one right: to your wages (and bonuses are not a right). Getting wages early isn't against any law I know of.
If I'm misunderstanding something, please clarify.

You got your pay, both the bonus and the end of year pay you talked about. They don't need to inform you of changes they made in paying a bonus. You got the pay you were entitled to, so what are you asking for?

Do you have a contract that spells out in detail when you are supposed to be paid? Did they explain the change to the other employees? If so, were you in the office at the time or away from it?

Did you read your pay stub to see exactly what it covered? If you still have your pay stub, read it and see what monies were paid-regular pay, pay period(s) covered, bonus if there was one. If it covered the Dec. 15th to Dec. 30th pay period, did you receive the correct amount?

So go back to your Dec 15 pay stub and check it. Make sure, that the gross salary was for a whole month. That's what pay stubs are for. I can understand not checking because it was larger than normal because of the bonus, but the stub should contain the information. There should have been separate line items for gross salary and gross bonus. If there was not, then you should go back to payroll and ask for an itemization.

Yes, you are entitled to pay from 12/15 to end of month. But, assuming that the pay stub lists the proper amount of salary for the month of December, then you already received it.

You were either paid, or not paid,if they paid early, there is no dispute possible, they may pay early if they want.

As noted, you go back and account for all of the money, your pay receipt should be specific as to what was pay and what was bonus