Non-Jews are actually forbidden to observe Shabbat (at least with regard to the prohibitions;
It depends on whether one follows the dictates of the Jewish tradition or not. I would say that biblically speaking those who join themselves to Israel through Christ become partakers of the blessings promised to Israel, including the blessing of the sabbath.
And there's where you jump the track.
"Biblically speaking" and "chr*stianity" simply don't go together.
Sometimes looking at modern Christianity and comparing it to the scripture it may seem that way.