Revelation 14:6 ASV
(6) And I saw another angel flying in mid heaven, having eternal good tidings to proclaim unto them that dwell on the earth, and unto every nation and tribe and tongue and people;
Because salvation is STILL from the Jews. John 4:22
Galatians 2:14 ASV
(14) But when I saw that they walked not uprightly according to the truth of the gospel, I said unto Cephas before them all, If thou, being a Jew, livest as do the Gentiles, and not as do the Jews, how compellest thou the Gentiles to live as do the Jews (Greek: judaize)?
Therefore, the inclusion of anything from the Gentiles (world) to the Jewish Evangel of the salvation of YESHUA leads not to His salvation. So, Christmas, Good Friday, Easter Sunday, catholic feasts, Sunday sabbath or Sunday worship, et cetera, may not be part of our judaizing in Yeshua unto our etrnal holy calling according to Judaism (Ephesians 1:4).
When we the Gentiles or the nations join YESHUA MASIYAH YAHWEH, we are assimilated into the unchangeable foundation of the commonwealth of Israel (Matthew 5:17-18). Ephesians 2:12 supports this eternal doctrine to fulfill Isaiah 56:8.
Ephesians 2:11-13 ASV
(11) Wherefore remember, that once ye, the Gentiles in the flesh, who are called Uncircumcision by that which is called Circumcision, in the flesh, made by hands;
(12) that ye were at that time separate from Christ, alienated from the commonwealth of Israel, and strangers from the covenants of the promise, having no hope and without God [ATHEISTS] in the world.
(13) But now in Christ Jesus ye that once were far off are made nigh in the blood of Christ.
Acts 15 does not include sabbath keeping because the Gentiles kept the eternal seventh day sabbath with the Jews as they judaize with the Jews every Sabbath day, according to Acts 15:21. Apostle Paul taught both Jews and Greeks in Corinth every sabbath (Acts 18:4), not every first day of the week.