Jubilees 6:36
35 For I know and from henceforth will I declare it unto thee, and it is not of my own devising; for the book (lies) written before me, and on the heavenly tablets the division of days is ordained, lest they forget the feasts of the covenant and walk according to the feasts of the Gentiles after their error and after their ignorance. 36 For there will be those who will assuredly make observations of the moon -how (it) disturbs the seasons and comes in from year to year ten days too soon. 37 For this reason the years will come upon them when they will disturb (the order), and make an abominable (day) the day of testimony, and an unclean day a feast day, and they will confound all the days, the holy with the unclean, and the unclean day with the holy; for they will go wrong as to the months and sabbaths and feasts and jubilees.
Mishnah Rosh Hashanah 1:4
MishnahOnly for the sake of two months may witnesses who saw the new moon desecrate Shabbat, should that be necessary in order for them to offer testimony before the court: For the month of Nisan and for the month of Tishrei, for in these months messengers are sent out to Syria, and by them, i.e., these months, the dates of the major Festivals are set: Yom Kippur, Sukkot, Passover, and Shavuot. And when the Temple was standing, the witnesses desecrated Shabbat for the fixing of the New Moon of all the months, due to the imperative of fixing the proper offering of the New Moon at the correct time.
Notes and References
"... The Interpolator’s insistence that human actions had had no role whatsoever in the creation of the Torah’s laws is of a piece with what he writes about the calendar. One calendrical system in use in Second Temple times was similar to the one used later, in rabbinic times: months were determined by the phases of the moon, with each month starting after two reliable witnesses had attested to having seen the new moon (see mishnah Rosh ha-Shanah, chapters 1-2). Such a system was a horror to the Interpolator, precisely because it depended on human intervention for the fixing of dates—not only the start of each month, but, as a consequence, also the occurrence of all God’s sacred festivals and other holy days within each month. Were not such things fixed in advance by God? He therefore has the angel of the presence warn Moses against “those who will examine the moon diligently” and use it to determine the beginnings of months (Jubilees 6:36) ..."
Kugel, James L. A Walk through Jubilees: Studies in the Book of Jubilees and the World of Its Creation (p. 14) Brill, 2012