Courts must calculate to determine when a new month begins

This mitzvah commands the courts (Sanhedrin) to determine and sanctify the new month based on calculations and testimony of the moon’s appearance. This practice established the Jewish calendar, ensuring that festivals and holidays were observed at their proper times. By linking sacred time to both human testimony and divine command, this mitzvah highlights the partnership between Hashem and Israel in sanctifying time.

Commentary & Classical Explanation:

• Rambam (Hilchot Kiddush HaChodesh 1:1–8): Explains that sanctifying the new month is a duty of the Sanhedrin and the foundation of the calendar.
• Sefer HaChinuch (Mitzvah 153): Emphasizes that the mitzvah ensures unity of observance, with all Israel celebrating festivals together.
• Talmud (Rosh Hashanah 20a): Details the procedures for witnesses of the new moon and the court’s role in declaring Rosh Chodesh.