The phrase, "day of the Sabbaths" is the usual idiom for the seventh day Sabbath.  We meet up with the phrase in the ten commandments of the LXX (Septuagint)  version:   mnhsqhti thn hmeran twn sabbatwn agiazein authn(Remember the day of the Sabbaths to make it holy, Ex. 20:8), and in the MT (Masoretic Text): 
 vwdql tbwh mvy ta rvkz
 (Remember the day of the Sabbath to keep it holy).   
 
day  of the Sabbath(s)
hmera(n) twn sabbatwn
 
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