I am searching for when the Roman calendar was mingled with the Hebrew calendar; By whom and how many? I know it had not been done yet when the new testament was written, because the days and months were still numbered and the Hebrew feasts were still kept.
That really is a great question that I would like to know the answer to as well. We know that the Gregorian calendar that we use today used to only have 10 months in it, ending in december (dec- meaning ten, sept- meaning 7, etc). We also know that the Roman Emperor Constantine invented the Catholic church around 323 CE and the council of Nicea really brought a lot of pagan things into the faith (easter, the date for easter, the theory of the trinity, etc). So it could quite possibly be around that time. We also know that the gregorian calendar was eventually changed to be a 12 month calendar, but we know the Hebrew calendar (not just for the Jews, but all the tribes) was always 12 months.
We also know that it was the council of Laodicea (363 CE?) when the Roman Catholic church assumed it had the power to change the Sabbath to Sunday (the day of the sun, the lord's day).
So to answer your question directly, I would say it was about 323 CE when Constantine invented the Catholic church as to when the Roman/Pagan/worldly calendar was mixed with the true Hebrew calendar. (Not "Jewish" @David Stratton. "Hebrew" involves more than just the one tribe of Judah)