” making a thirteen day difference, so December 25 inclusive becomes January 6″
The Orthodox church celebrates January 7th the 13 days are added Dec 25 + 13 = Jan 7
” After 2100 it will change to January 5.”
It will change to January 8. 2100 Will not be a leap year on the Gregorian Calendar it will be on the Julian Calendar. so that Gregorian year will be 365 days long that Julian year wil be 366 days long. This will make the difference between the 2 observances 1 day greater, not less.
Lehavdil ad lanetzach, The difference between the Gregorian date and 60 days after the Tekufas tishrei as calculated by Shmuel (which h corresponds tho the Julian Calendar) will also grow by 1 day causing vesen tal umatar to be recited on Dec 5/6 instead of the current 4/5
(there are some groups Amish, Mennonite that celebrate Jan 6, but as these groups were not existent “in di heim” nobody that I am aware of keeps nittel based on their date)
“It’s not about the Goyishe celebration. It’s about Tekufas Teves.”
I have never heard any source tying the Tekufa to Nittel. Some observe nittel Dec 25, Others Jan 7. Tekufas Teves according to Shmuel is Jan 6 , based on Rav Adda it is Dec 26
charlie