The 25th of Adar begins the ninth set of 22 days of the Jewish calendar, which parallels the letters Peh and Tzadik, as well as the Thick and Thin Clouds in Perek Shirah. This 22-day period begins during the time of Passover preparations, and runs through the first days of Passover.
Peh stands for Pedut, redemption. Peh literally means "mouth," which is one of the primary symbols of Passover itself. Rabbi Levi Yitzchak of Berditchev states that Pesach stands for Peh - Sach, a mouth converses, as it is a special mitzvah to recount the miracles of our liberation during the Passover Seder. The Megaleh Amukos states that Pharaoh stands for Peh Rah (evil mouth). (Raskin)
The shape of the letter Peh is a combination of the Kaf and a Yud. The Yud stands for G-dliness, and therefore the Peh symbolizes the revelation of G-dliness. Also, the Yud is like a tooth inside the mouth. In the Haggadah of Passover, we learn that the we must "break the teeth" of the evil son, so that he may become a Tzadik, revealing his potential.
The next letter, the Tzadik, stands for righteousness. The Tzadik is also a combination of letters: a bent Nun and a Yud. The bent Nun symbolizes humility, the trait most associated with Moshe, of whom the Torah testifies that he was the most humble of men. Humility, destroying one's spiritual Chametz (leavened bread, ego) is certainly one of the main themes of this time as well. It is also through humility that we reveal the Yud, G-dliness.
A similar theme can be found in the Perek Shirah verses of the Thick and Thin Clouds:
The thick, usually dark, clouds symbolize the potential for tremendous rain. This overflow of water (which is always a metaphor for the Torah itself) is very much connected to extensive recounting of the Passover story, as well as the potential the evil son has to bring tremendous good and blessing to the world.
The light clouds also spread the knowledge of G-d, "scattering His light." The clouds' lightness also appears to symbolize humility. It is their very lightness that allows them to have such strong reflective powers. The same is true for all Tzadikim and the Jewish people as a whole. They are a reflection of G-d's light.
In Hebrew, light clouds are called Ananei HaKavod, "Clouds of Glory," the term used for the clouds that protected the Jewish people during the the time of the Exodus from Egypt and the Passover story. The clouds were given to the Jewish people in the merit of Aharon's righteousness. After Aharon passed away, the clouds disappeared. They were later reinstated in the merit of another Tzadik, Moshe Rabbeinu.