Objection: The Luni-solar advocates rely on the false claim that there was a period of Jewish history when the timing of the Sabbath was forgotten. Aside from lacking historical evidence, the idea is irrational. The timing of the Sabbath can be forgotten only if it is not tied to natural phenomena, such as the moon. If they were correct, the timing of the Sabbath could not be forgotten.
Rebuttal: If there are luni-solar calendar advocates that claim there was a period of Jewish history when the timing of the Sabbath was forgotten, WLC is not among them. With the single exception of their bondage in Egypt, WLC does not believe that the Jews “lost” the Sabbath. (Scripture itself establishes that the original calendar was restored to the Israelites at the time of their deliverance from Egypt.) However, by their own admission, the Jews deliberately and knowingly set aside their Creator-given calendar by which the Sabbath was calculated. According to David Sidersky, “It was no more possible under Constance to apply the old calendar.” “Under the reign of Constantius (337-362) the persecutions of the Jews reached such a height that . . . the computation of the calendar [was] forbidden under pain of severe punishment” (excerpted from The Jewish Encyclopedia, "Calendar.")
After the Council of Nicæa legislated the use of the Julian calendar for all religious observances, persecution of the Jews and apostolic Christians was so intense that the last president of the Sanhedrin in Judea, Hillel II, developed a new method of time calculation that would allow Jews to blend with Romans using the Julian calendar. “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 [4th century A. D.], the last President of the Sanhedrin, the Romans prohibited this practice. Hillel II was therefore forced to institute his fixed calendar, thus in effect giving the Sanhedrin's advance approval to the calendars of all future years” (“The Jewish Calendar and Holidays (incl. Sabbath): The Jewish Calendar: Changing the Calendar,” www.torah.org.)
The probable rationalization for this move was the tradition, taught in the Talmud, that there is no sin for breaking the Sabbath provided one does not know when the Sabbath occurs. Under such circumstances, so the tradition of the rabbis taught, all one had to do was to keep one day in seven. By instituting a “fixed” calendar, Hillel II used his authority as president of the Sanhedrin to change the calendar. It is correct that if people had continued to use the moon for establishing times of worship, as the Bible teaches (see Psalm 104:19; Leviticus 23; Genesis 1:14), the Sabbath would still be correctly kept by the faithful.
There was no period of history that “lost” the Sabbath. Rather, by a deliberate and knowledgeable act on the part of the last president of the Sanhedrin, the Biblical calendar was set aside and another substituted in its place. Thus, the common people were not told when the true Sabbath came, but it was a deliberate act by those who did know. Maimonides and most other Jewish chronologers agree that the modern Jewish calendar is based upon the "mean motions of the sun and moon, the true having been set aside." (Maimonides, Kiddusch Ha-hodesch, Tr. Mahler, Wein, 1889.)