Primitive Christianity is a movement that attempts to practice the exact “faith once delivered to the Saints” (Jude 1:3). That is to say, the doctrines and practices that Jesus conveyed to the first apostles without changes, embellishment or contamination by people who thought they knew better.
Such Christians observe the same Sabbath that Jesus observed, they keep the same Holy Days that Jesus kept and no others and they do His commandments fully, not adding to them (Revelation 22:14). They would also utterly reject the observation of any syncretic days such as Halloween or Xmas.
see for example the book by Alan Knight, Primitive Christianity in Crisis.
Additionally, and more to your question, see:
Study Topic: Is God really a Trinity?
By John H. Ogwyn
http://www.lcg.org/cgi-bin/lcg/studytopics/lcg-st.cgi?category=FalseReligion1&item=1116550111
Ok, so no Christmas or Easter and no Sunday Holy Day but Sabbath?
That is to say, the doctrines and practices that Jesus conveyed to the first apostles without changes, embellishment or contamination by people who thought they knew better. -- please clarify, do you mean also none of St. Paul's teachings?