THE TRUE SABBATH

I will also cause all her mirth to cease, her feast days, her new moons, and her sabbaths, and all her solemn feasts. - Hosea 2

The New Moon is still, and the Sabbath originally was dependent upon the lunar cycle...
Originally, the New Moon was celebrated in the same way as the Sabbath; gradually it became less important while the Sabbath became more and more a day of religion and humanity, of religious meditation and instruction... ("Holidays", Universal Jewish Encyclopedia, page 410)

Thus saith the Lord GOD; The gate of the inner court that looketh toward the east shall be shut the six working days; but on the sabbath it shall be opened, and in the day of the new moon it shall be opened. Ezekiel 46:1

And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, saith Yahovah. Isaiah 66:23

The months of the year were lunar and began with the new moon. (Universal Jewish Encyclopedia, page 631)

Declaring the new month by observation of the new moon, and the new year by the arrival of spring, can only be done by the sanhedrin. In the time of Hillel II [ 4 th century AD], ... the Romans prohibited this practice. Hillel II was therefore forced to institute his fixed calendar. ("The Jewish Calendar: Changing the Calendar," torah.org The Judaism Site)

"The present Jewish calendar was fixed in the fourth century." (Rabbi Louis Finkelstein-Jewish Theological Seminary of America)

The modern Jewish calendar is based upon the "mean motions of the sun and moon, the true [calendar] having been set aside." (Maimonides, Kiddusch Ha-hodesch)
And he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time. Daniel 7:25