Anciently, calendars were not available to everyone the way they are today. In Israel, the knowledge of the calendar and its computations was entrusted to the Levitical priesthood. It was their responsibility to compute the beginning of each month, the new moon, and make it known to the nation.
A ceremony took place on the day of the new moon to let the people know that a new month had begun. In this manner, the general populace was kept aware of God's calendar so they could prepare for any approaching seasons, festivals,
Today, it is not necessary to mark the beginning of each new month with an assembly. Calendars are widely distributed, and anyone can determine the beginning of a new month simply by looking on a calendar displaying that information.
On the other hand, there is no biblical prohibition against observing the new moon. It is entirely a personal matter.