Tabernacles should always happen in October (Or at least after the Fall Equinox).

I recently became aware that among Hebrew Roots type Christians there is a common opinion that the current official Rabbinic Jewish Calendar was doing everything a month late during 2024, or rather 5785.

I have express skepticism of the very concept of using a Lunar based Calendar.  But if we are going to use a Lunaisolar Calendar with a potential 29-30 day margin of error on when to the year, there is good Biblical reason why airing on the side Caution means starting it later rather then sooner.

Deuteronomy 16:13 is clear that The Feast of Tabernacles (The 15th-21st day of the Seventh Month) is held AFTER the harvesting of the Corn and Wine is done, not during, after, that's also why it's called Ingathering in it's two Exodus references.  This is also shown by Leviticus 23:39-40.  The Grape Harvest extents through the entirety of September even potentially into early October.

So those who say the Hebrew Calendar is an entire Lunar Month late this year would be having Tabernacles start in the middle of September when the Grape Harvest is far from over.

The completely non Lunar Equinox based system I think should be used would be to begin the year the day after the Spring Equinox, which currently most often happens on March 20th but in antiquity often fell a bit later.  Starting the Torah year on March 21st presuming 30 Day minimum months with no Intercalary days in the first 7 months places the first day of Tabernacles on October 1st (and the Feast of Jeroboam on October 31st).

But perhaps even later then that is best to be safe. March 25th is the Feast of the Annunciation because ancient Greco-Roman Christian considered that day the Spring Equinox even if that wasn't usually astronomically correct.  That would begin the Seventh Month on the 21st of September and place Tabernacles on October 5th-11th with the Eighth day as October 12th.

I firmly believe that the origins of Michaelmas are in part early Christianization of one of the Seventh Month Holy Days, but which one is complicated to determine.

Michaelmas is currently officially September 29th, but there is evidence it was originally September 30th with the 29th being the Eve of Miachaelmas and thus a day a certain Church Building was consecrated in preparation for the Feast.

Michaelmas is largely viewed as a Feast Day in the strictest sense linked to the Fall Harvest and thus would fit being Tabernacles.

But the traditional Scripture Readings for Miachaelmas include Revelation 12:7-12 the account of Satan being cast out of Heaven.  And I see logic to associating that with Yom Kippur and the casting out of the  Azazel goat in Leviticus 16.  Plus the complicated relationship between September 30th and the 29th could parallel the complicated relationship between the 10th and 9th of the Seventh month in determining  Yom Kippur in Leviticus 23.