This week, instead of reading the regular Parshas Tzav Haftorah, we read the Haftorah for Parshas Parah (Yechezkel 36:16-38). Maftir discusses the purification process of the Parah Adumah (red cow), executed when someone became ritually impure through contact with a corpse.
When the Bais Hamikdosh stood, it was instituted to be read yearly on the last Shabbos before Rosh Chodesh Nisan. This served as a reminder for those living far from Jerusalem to prepare to journey to the Beis Hamikdash to purify themselves in time for the Korban Pesach.
The Haftorah describes a prophecy of Yechezkel that also refers to the purification process: "And I shall sprinkle pure water upon you, that you be cleansed."
The Levush (685:1) notes that on this Shabbos, one should pray that Hashem will quickly sprinkle the pure waters on us, may it be soon.