Final answer:
Parshat Hachodesh is read on the Shabbat before the Jewish month of Nissan or on the first of Nissan if it coincides with Shabbat, in preparation for Passover.
Step-by-step explanation:
Parshat Hachodesh is traditionally read in the synagogue on the Shabbat before the Jewish month of Nissan or the actual first of Nissan if it falls on Shabbat. It includes reading Exodus 12:1-20, which details the laws of the Passover offering and the mitzvah (commandment) to sanctify the new moon. This passage is part of the preparations for Passover, the Jewish holiday commemorating the Exodus from Egypt. The timing is significant because Passover begins on the 15th of Nissan, and reading Parshat Hachodesh serves as a precursor to the Passover celebrations.