The only testimony that is acceptable with regard to [the sighting of] the new [moon] is that of two adult males who are fit to testify regarding all matters...
When a father and a son both see the new moon, they should both go the court to testify. Not because testimony regarding the [the sighting of] the new [moon] is acceptable [from witnesses who are] related, but because one of them may be disqualified because he is a thief or for other reasons, and the other will be able to join with another person and give testimony.
Rambam, Laws of Sanctifying the Month 2:1
The law is that although we already know when the date for the new month is according to mathematical calculation, one must still have the testimony of two proper witnesses to establish the day of Rosh Chodesh. The Torah wished that there be the standard testimony that causes that “the matter shall be confirmed,” and not merely an account of two trustworthy individuals. We see this expressed in the following Talmudic statement.
[The verse states] “This month is for you the head of months”, [which implies], See [the moon] like this and then sanctify.
Talmud, Rosh HaShana 20a
In order to sanctify the new moon, it is not enough merely to know the proper date of the new month, one must also witness it and bring that testimony to a court through the process of authentic testimony. The Rabbi’s understood from this that although they knew the date, the Torah still necessitated authentic testimony to establish the day of the new month.
