I work for a company who decided to grant this past Monday (July 3rd) a holiday, in addition to the normal July 4th holiday. I came into work on July 3rd. I was told after the fact that I will not receive overtime pay on the 3rd and working that day is now considered on my own time. Am I entitled to overtime pay for working on the 3rd?

OT for working holidays is a matter of company policy.

working that day is now considered on my own time
You still need to be paid for all hrs you work regardless of the day.

Is the company allowed to make the argument that I was not authorized to work on the 3rd even though I was never explicitly told I could not?

They can apply discipline, up to and including termination, for working while not authorized, but they still have to compensate the worker.