• The Bible does not define what a new moon is, so we are not instructed how to watch for the new moon. This argument is made in support of substituting the astronomical conjunction for the actual sighting of the new moon. Let’s look again at Deuteronomy 16:1, paraphrased as closely to the Hebrew meanings as Strong’s dictionary allows. “Look narrowly for the new moon of the green ears of grain and keep the Passover.” Once again, Moses was speaking to all of Israel here. He told them to look for the new moon of Abib. He did not tell them to check with the Levites about molads. A molad (conjunction) as we have already learned, is when the moon is exactly between the earth and the sun. This argument substitutes the molad, a moon you cannot see, for the new moon crescent, which you can. https://yrm.org/wp-content/uploads/2016/10/The-Biblical-Calendar-updated.pdf
4 Lie thou also upon thy left side, and lay the iniquity of the house of Israel upon it: according to the number of the days that thou shalt lie upon it thou shalt bear their iniquity.
5 For I have laid upon thee the years of their iniquity, according to the number of the days, three hundred and ninety days: so shalt thou bear the iniquity of the house of Israel.
6 And when thou hast accomplished them, lie again on thy right side, and thou shalt bear the iniquity of the house of Judah forty days: I have appointed thee each day for a year.
7 Therefore thou shalt set thy face toward the siege of Jerusalem, and thine arm shall be uncovered, and thou shalt prophesy against it.
8 And, behold, I will lay bands upon thee, and thou shalt not turn thee from one side to another, till thou hast ended the days of thy siege.
9 Take thou also unto thee wheat, and barley, and beans, and lentiles, and millet, and fitches, and put them in one vessel, and make thee bread thereof, according to the number of the days that thou shalt lie upon thy side, three hundred and ninety days shalt thou eat thereof.
As far as I know this is the only place in the Scripture which provides evidence of an intercalary month. Contextually, this shows that during the captivity period the Jewish people understood time in the sense of an intercalated lunar/solar calendar. Their religious calendar was normally 12 lunar months of 29.53 days each followed by an intercalary or 13th month every 2 or 3 years.
In a symbolic sense the rising and setting of the sun regulated man’s labor and rest throughout the solar year. The lunar cycle on the other hand regulated the religious calendar and its mow’eds (appointed times). These two cycles were synchronized by the 13th Month.