New Moon occurs at 1 PM in Israel, 17th of Sept., which is the 29th day of the 6th Month;
Meaning the sunset a few hours later is the start of the Seventh Month which is Yom Teruah (Feast of Trumpets):
Leviticus 23:23-25 YLT
This lands Yom Kippur, the 10th day of the 7th month on Sept 27. However, the rabbis decided (man made traditions) that Yom Kippur must never land on a sunday, with back-to-back Shabbats. That’s why they moved the first day of the month to Sept 19 this year (beginning at sunset near the end of the 18th). This rule is nowhere in the Bible, so we leave Yom Kippur exactly where it lands, on Sept 27.
This is why it is so good to be FREE to follow the Word! It is a light burden, without all the added-on traditions of men.