Why St. Gregory Palamas is a Sabbath keeper

No, he didn't observe the Saturday  Judaic ordinance.  Rather,  in the biblical and patristic sense,  Christians observe the Sabbath.  Not just on  Sunday,  but every moment,  in fact,  every breath.  This is how  St.  Gregory interprets the Sabbath.  The excerpt is from  question 2,  his first Triad to a young monk beset with  bewilderment.

The Sabbath was a command to cease from work. So serious was it,  one could be stoned for violation, as did occur.  The Sabbath, doctrinally, for Christians is that Christ said "it is finished."  He has reconciled man to God  by changing human nature and deifying it. But we participate in the Sabbath  by ceasing from our own  works and thoughts.  And this makes sense.  If the last act we will ever do  is give up our breath, and enter eternity,  then  we should consciously lives reflecting this.

Consequently then, for those recently stripped down  for this contest and gathered in their mind, they continually start off for this race,  but they also need continually again to return to this practice,  as the mind eludes those who are unexercized, since it is the hardest thing to look at and moves more than anything.  Because of this, they  encourage to pay attention to the frequent scattering of the mind and  return again to oneself in inhalation and to hold it back for a little, keeping that guardedly in this (breathing) until by God’s help progressing to the better, and without much toil, having made his  own mind  unable to approach to things about him, he may be able to exactly gather into “a singular form collected.” ....this automatically occurs following the attention of the mind. For there is quietude by this entering in and going out -the spirit upon every ingoing thought, but especially in the case of those who practice stillness in body and reasoning. For those who spiritually sabbatize are these who also  rest from all their works, because it is actually easy. Everything discursive and produced by discursive reasoning and which has been elaborately reasoned about knowledge of the powers of the soul  they completely strip away, namely the work and all the aids for aiding bodily feelings, helps and quite simply every bodily activity,  which is in our power, but not in our power, even the end, exactly as respiration, so long as it is in our power. All these identical things  follow painlessly and without much care for those who make progress hesychastically. And of necessity, perfectly automatically all things occur for by same entrance of the soul to itself.