Modern Orthodox Jews will tell you, "Of course we know what day that is; it's marked on our calendar." It was not always this way.
"Originally the New Moon (for the Feast of Trumpets) was not fixed by astronomical calculations, but was solemnly proclaimed after witnesses had testified to the reappearance of the crescent of the moon.
On the 30th of each month, the members of the High Court assembled in a courtyard in Jerusalem, named Beit Ya'azek, where they waited to receive the testimony of two reliable witnesses; they then sanctified the New Moon. If the moon's crescent was not seen on the 30th day, the New Moon was automatically celebrated on the 31st day.
This, therefore, is a festival that, "No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father."
The Meaning of Rosh Hashanah
The Feast of Trumpets is the only Jewish Holiday that is celebrated on the first day of the lunar month (new moon).
"Originally the New Moon (for the Feast of Trumpets) was not fixed by astronomical calculations, but was solemnly proclaimed after witnesses had testified to the reappearance of the crescent of the moon.
On the 30th of each month, the members of the High Court assembled in a courtyard in Jerusalem, named Beit Ya'azek, where they waited to receive the testimony of two reliable witnesses; they then sanctified the New Moon. If the moon's crescent was not seen on the 30th day, the New Moon was automatically celebrated on the 31st day.
This, therefore, is a festival that, "No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father."
The Meaning of Rosh Hashanah
The Feast of Trumpets is the only Jewish Holiday that is celebrated on the first day of the lunar month (new moon).