Jeroboam changes the time of worship:
And Jeroboam ordained a feast in the eighth month,
on the fifteenth day of the month,
like unto the feast that is in Judah,
and he offered upon the altar.
1 Kings 12:32
The fall festival was held during the 7th month & Jeroboam changed it to the 8th.
Hmmm.....the 7th day is the Sabbath, yet many want it celebrated on the 1st day (or as many Christians refer to it as-the 8th day....since you had Sunday thru Sabbath of redemption week & then He rose on the 1st day, also called the 8th day, to begin the new creation).
"For in respect of the observance of the eighth day of the Jewish circumcision of the flesh, a sacrament was given beforehand in shadow and in usage; but when Christ came, it was fulfilled in truth. For because the eighth day, that is, the first day after the Sabbath, was to be that on which the Lord should rise again, and should quicken us, and give us circumcision of the spirit, the eighth day, that is the first day after the Sabbath, and the Lord's Day, went before in the figure; which figure ceased when by and by the truth came and spiritual circumcision was given to us."
—Cyprian, Letter LVIII
"The number 8 was, for ancient Christianity, the symbol of the Resurrection, for it was on the day after the Sabbath, and so the eighth day, that Christ rose from the tomb. Furthermore, the seven days of the week are the image of the time of this world, and the eighth day of life everlasting [this understanding was also that of the Jews]. Sunday is the liturgical commemoration of this eighth day, and so at the same time a memorial of the Resurrection and a prophecy of the world to come. Into this eighth day, inaugurated by Christ, the Christian enters by his Baptism." Cdl. Danielou quotes St. Ambrose, Bishop of Milan who died in A.D. 397
I-N-T-E-R-E-S-T-I-N-G
Just like Jeroboam changed worship from the 7th to the 8th month, those who brought spiritual "Egypt" into the church long ago introduced the 1st/(8th) day as the day of worship in place of the 7th day Sabbath.