Leviticus 23:23-25
23 The LORD said to Moses, 24 “Say to the Israelites: ‘On the first day of the seventh month you are to have a day of sabbath rest, a sacred assembly commemorated with trumpet blasts. 25 Do no regular work, but present a food offering to the LORD.’ ”
It was the only new moon day God called a Sabbath and commemorated it with sacrifices and trumpet blasts. It is called 'The head of the year' because it is New years day because it was when God made the Earth etc, and was also the day Jesus was born!
It is one of the 7 most Holy days. You are very much mistaken.
Even your own statement is not factually correct because Lev23:24 says shabbaton, which is a rest, but not necessarily a Shabbat, while at the same time Yom Kippurim, (the tenth), is called more critically and importantly a Shabbat shabbaton, that is, a Shabbat or Sabbath of shabbaton-rest. Not even modern Judaism keeps it this way, so I suppose both you and Jacob will disagree with this, but this is what the passage says and thus teaches, (Lev23:24-32).
How therefore can you have a Shabbat on the first and a Shabbat on the tenth in the same month? Is this another place where you insert days that do not even actually get counted in your calendar? There is no way even this one little technicality will work on your calendar because if you insert extra days you must insert them at the end of the previous month, between the months: but there is no way to insert your extra days between the first and the tenth of any given month and there certainly is no way to remove any days. I understand why some will add days to keep the Shabbat on the 8th, 15th, 22nd, and 29th in their calendar, but I have never heard of anyone trying to add or remove days between the first and the tenth of a month or anywhere in the midst of an ongoing month. And obviously you cannot have a Shabbat on the first and a Shabbat on the tenth if you have a seven day week consisting of seven twenty-four hour days: it's that simple, and yet Yom Kippurim is plainly stated to be a Shabbat shabbaton, (Lev23:32), which is a Shabbat of rest.
And since you claim to be the all-knowing chief kahuna when it comes to calendars you should have already known about this little technicality, eh kahuna? Why therefore do you say that Rosh Hashanah is called a Sabbath when it is not? A shabbaton is not always a Shabbat because the Shabbatot are called Shabbat shabbaton beginning in Exo16:23, (which also calls it holy, a shabbaton Shabbat kodesh). Other days which are not Sabbaths, such as Rosh Hashanah, can still be called a shabbaton-rest but that does not mean they are the same as the Shabbat.