geoffreycaveney's Judaeo-Greek theory

[Markus]

 The Septuagint (translation of old testament into Greek, 2nd century BC) has ἑβδομάζω meaning to keep the sabbath, and the epistle to the Hebrews 4.4 has ἡ ἑβδόμη meaning the sabbath. So the usage has both old and new testament precedents.

Edit: ground also covered by Geoffrey's simultaneous post above.