When does the Biblical month begin?

[Guide To The Bible]

According to these verses it starts with the sighting of the new moon:

1 Samuel 20:18 + 24-27
18 Then Jonathan said to David, “ Tomorrow is the New Moon feast [Day1]. You will be missed, because your seat will be empty.

24 So David hid in the field, and when the New Moon feast came [Day1], the king sat down to eat. 25 He sat in his customary place by the wall, opposite Jonathan, and Abner sat next to Saul, but David’s place was empty. 26 Saul said nothing that day, for he thought, “Something must have happened to David to make him ceremonially unclean,20 surely he is unclean.” 27 But the next day, the second day of the month, [Day2] David’s place was empty again. Then Saul said to his son Jonathan, “Why hasn’t the son of Jesse come to the meal, either yesterday or today?”

This had to be the visible new moon which can be seen by the naked eye when there is just enough light on the moon to be seen as the first crescent of the waxing moon. It could not be the astronomical new moon as this cannot be seen by the naked eye.

However, the modern Jewish calendar sometimes has the month beginning on the astronomical new moon, such as today, the 16th of February, as can be seen in this link: Hebrew Date Converter - 1st of Adar, 5778 | Hebcal Jewish Calendar

But as you can see in this link the astronomical new moon is on the 16th of February:

lunar month february 2018 - Google Search:

The reason why this is so is because the modern Jewish calendar was invented by Jewish Rabbi Hillel II in 359 AD and he used the 19 year repeating lunar cycle known as the Metonic cycle, (named after the Greek astronomer Meton of Athens) to approximate when the observable new moon is but it is only correct about half the time.

This proves that the modern Jewish calendar is man made and was only designed to replicate God's original Calendar that He gave to Moses and which Jesus also used. The reason why this is important is because all of the feasts Jesus fulfilled were calculated using God's original Calendar, such as:

Passover = Easter
Shavuot = Pentecost
Tabernacles = Harvest Festival

These were the three most important feasts in the year:

Deuteronomy 16:16
Three times a year all your men must appear before the LORD your God at the place he will choose:

1. Passover = Easter - FULFILLED BY JESUS
2. Shavuot = Pentecost - FULFILLED BY JESUS
3. Tabernacles = Harvest festival -YET TO BE FULFILLED ?

 

[Sabertooth]

Tabernacles = Harvest festival -YET TO BE FULFILLED ?

There is strong Biblical math that puts Jesus' birth at the Feast of Tabernacles. If true, that means that it was fulfilled in The Incarnation (as in "tent" = "body").

 

[Yekcidmij]

The reason why this is so is because the modern Jewish calendar was invented by Jewish Rabbi Hillel II in 359 AD and he used the 19 year repeating lunar cycle known as the Metonic cycle, (named after the Greek astronomer Meton of Athens) to approximate when the observable new moon is but it is only correct about half the time.

This proves that the modern Jewish calendar is man made and was only designed to replicate God's original Calendar that He gave to Moses and which Jesus also used.

What makes you think the calendar in use in the first century was the "correct" one? There were Jews who believed it wasn't (eg, the Qumran sect).

 

[SAAN]

I dont think the sliver of the moon the Jews use is correct, as they would always be 3 days late trying to calculate a new moon, waiting for the sliver of the dark moon.