Shabbat Parah ("Sabbath [of the] red heifer" שבת פרה) takes place on the Shabbat before Shabbat HaChodesh, in preparation for Passover. Shabbat HaChodesh falls on 21st March which corresponds to 25th Adar.
Numbers 19:1-22 (Parashat Chukat) describes the parah adumah ("red heifer") in the Jewish temple as part of the manner in which the kohanim and the Jewish people purified themselves so that they would be ready ("pure") to sacrifice the korban Pesach.
“This is the decree of the Torah, which Hashem has commanded, saying: Speak to the Children of Israel, and they shall take to you a completely red cow, which is without blemish, and upon which a yoke has not come.” (BaMidbar 19:2)
This is an example of Law of the Torah which is considered completely above human comprehension. The paradox is that those who are involved in the preparation of the ashes of the cow become ritually impure, while the sprinkling of water with those ashes is used to remove contamination! It is an example of a Law which must be accepted on faith alone and is known as a chukah, a law that defies human logic and understanding.