@Meadmaker, #458
You accept that Hillel [II] reformed the Jewish calendar in 358/359 AD. Why did he reform it? What did he do?
See Svenax' answer.
It was all about finding a way to determine when Nisan 1 was, so that the holidays could be celebrated at the proper time.
In modern Jewish custom, yes, Passover is celebrated once a year only on Nisan 14 but prior to the Jewish lunisolar calendar’s reformation in 358/359 AD, Passover was in a purely lunar calendar that is why Constantine wrote “in their [the Jews’] blindness and repugnance to all improvements, they frequently celebrate two passovers in the same year. We could not imitate those who are openly in error.” Why two passovers in the same year?
Oh, please. That's the exact opposite of what I said.
In modern times, Passover is celebrated twice per year. It is celebrated on Nisan 14, and it is celebrated on Nisan 15.
Really. It is. I'm not making this stuff up.
Some time during the 19th century, the rabbis in the Reform movement decreed that this was an unnecessary practice, so Reform Jews usually only celebrate once. For those of us who are part of a Reform synagogue, the second seder is considered an option. Very commonly, the first seder, on Nisan 14, is more formal with a more elaborate meal, while the second seder is done with faster haggadah reading and might be a potluck. In our family, we always get together with family on Nisan 14, and sometimes we get together with friends on Nisan 15.
Among Conservative and Orthodox Jews, though, the custom is to celebrate two full Passover celebrations every year. There is an exception to this, though. Jews living in Israel celebrate only one Passover.
Why?
The custom dates back to ancient times, before the time of Constantine in fact. The first day of the month was determined by the new moon at Jerusalem. They weren't as good at astronomy back then as we are now. Given a date of one new moon, you could know that the next new moon would be either 29 or 30 days later, but you weren't sure which. In order to determine when the first day of the month occurred, they had to look at the sky. The priests would watch the sky and declare that yes, it was a new moon tonight, and would proclaim it to all the people.
That worked fine for people living nearby, but what about people in Babylon and elsewhere? For the Babylonian Jews, they actually set a series of signal fires stretching across the desert. However, the Samaritans thought it was funny to light fake signal fires and confuse the Jews, so they started dispatching runners to Jewish communities in Babylon and around the Mediterranean. This took a long time, so they were never absolutely, 100% sure, during the first half of the month, what day it really was. Just to be absolutely certain they were getting it right, some Jews celebrated the holidays twice, every year. This practice eventually made its way into the Talmud, and became the normal way of doing things. Even though we now can be absolutely certain when Jerusalem will have a new moon, the custom survives to this day.
That is why Constantine said that the Jews sometimes celebrated Passover twice in one year. Because they did. We still do. It's no secret and requires no special insight.
But we never celebrate Passover in December, and there's no record of any Jew ever having done so.