Parshas Parah is read on the Shabbos before we read Parsha Chodesh. This is because right after the Mishkan was built, we burnt the red heffer in order to get it’s ashes so that we could purify ourselves before the Korban Pesach. There are different opinions whether this reading is commanded by the Torah or if it is Rabbinically commanded.
To explain where in the Torah it may have been commanded, the Yalkut Gershoni says that really the main commandment is to remember about the Golden Calf. The Magen Avraham (685:2) says that so not to publicly discuss the downfalls of the Klal Yisrael, we don’t read the parsha of the Golden Calf. Instead, we read about the red heffer, which is an atonement for the Golden Calf, in order to fulfill the mitzvah of remembering the Golden Calf.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
If you have any questions or comments please send them to:
[email protected]