The Four Special Torah Readings
2. On the Shabbos before Purim, we read "Parashas Zachor" (Deuteronomy
25:17-19) (1). On the Shabbos before Rosh Chodesh Nissan, we read "Parashas
HaChodesh" (Exodus 12:1-20) (2). If Rosh Chodesh Nissan falls on Shabbos,
"Parashas HaChodesh" is read on that day, according to the sequence
outlined [for the reading of "Parashas Shekalim"] when Rosh Chodesh Adar
falls on Shabbos.
On the Shabbos before [we read] "Parashas HaChodesh" we read "Parashas
Porah" (Numbers 19:1-22) (3).
(1) This paragraph states the commandment to recall the attack of the
nation of Amalek which occurred just after the exodus from Egypt.
(2) This paragraph describes the mitzvah the sanctify the new moon and the
mitzvah to offer the "Korban Pesach" (Passover offering) on the afternoon
of the fourteenth of Nissan.
(3) "Parashas Porah" describes the purification ceremony of those who are
ritually impure due contact with a human corpse.