No.
I'm fairly certain that Pagans do not, and never have, celebrated the birth, life or death of Christ, so I think it's a safe bet that "Christmas" was not a pagan holiday. I don't know much about pagan holidays, but I thought that their holiday was "yule" and had to do with honoring the winter solistice.
Yeah - Christians moved their Christmas celebration to the solstice, and adopted some of the more festive traditions of the pagans. But I don't see how that dilutes or invalidates the reason behind Christmas though.
Technically, we are celiebrating Christ's birth.
We're just not doing it on His actual birthday. I don't think it's the actual date which is important, but the event itself.
At my last job we celebrated Martin Luther King Jr's birthday the Friday after Thanksgiving.
Big whoop.