Law 5

Sighting of the moon is not up to an individual, unlike determination of the Shabbos, where one counts 6 days and observes Shabbos on the 7th, but is given to the court. Only when the court sanctifies and establishes the day as the beginning of the [new] month does it become the beginning of the [new] month. As it is said: "this month shall be for you" - this testimony is entrusted to you.