Just to clarify my own posts [previous post in this thread], in case it wasn't clear to the readers
, I believe the sabbath is always Saturday, and "Firstfruit" (Lev23:10-12) is always a Sunday (and was the day of Jesus' Resurrection).
As to the day Jesus was born, I believe a reasonable case can be made (based on Scripture) that it was likely Tishri 15 (not Tishri 8, which another poster suggested), that is, the first day of Tabernacles (Lev23:34) [and then circumcised the 8th day; "the eighth day" mentioned in Lev23:36], ascertained by viewing the "courses" of the priests (during those days [NT passage]) and that John the Baptist was born 6 months earlier. Also the wording in John 1:14, "and the Word was made flesh, and
tabernacled among us"... and I too believe that the purpose of the Zech14 (future) celebration of Tabernacles is due to this (His BD party, so to speak, lol).
I too believe we are free to celebrate it (pr not) at any time