sabbatismos-keeping sabbath

"There remaineth therefore a rest(sabbatismos) to the people of God."

There are many Christian groups who claim that there is no evidence of Sabbath worship in the New Testament. But Hebrew 4:9 clearly states that there remaineth a keeping sabbath to the people of God.

The next verse clearly defines the sabbatismos this way. "He that enters into his rest, he also hath ceased from his own works as God did from his."

hmmm which day did God rest?