According to one account in the Midrash, on the 13th of Nissan of the year 2048 from creation (1714 BCE), G-d appeared to Abram, changed his name to Abraham ("father of a multitude of nations") and commanded him to circumcise himself and all members of his household--and all future descendents at the age of eight days--so that "My covenant (brit) shall be in your flesh, as an eternal covenant." Abraham was 99 years old at the time, and his son Ishmael, 13. (Isaac, who was born a year later, was the first Jew to be circumcised at eight days).

Link: More on the Covenant of Circumcision

In the 12th year of his reign (357 BCE), King Achashverosh of Persia endorsed Haman's plan "to destroy, kill and annihilate all Jews, from young to old, infants and women, on a single day, on the 13th day of the 12th month, the month of Adar." On Nissan 13 (11 months before the date chosen for the massacre) proclamations of the decree were drafted and dispatched to all 127 countries of the Persian Empire. Mordechai told Esther to go before the king and plead for her people. Esther asked that a three-day fast be proclaimed (Nissan 14, 15 and 16--including the first two days of Passover) in which all Jews would repent and pray for the success of her mission.

Links:
The Story of Purim
Timeline of the events connected with the Purim miracle

Nissan 13 is the anniversary of the passing of Rabbi Joseph Caro (1488-1575), author of the Shulchan Aruch ("Code of Jewish Law").

Link: Original Ideas

The third Rebbe of Chabad, Rabbi Menachem Mendel Schneersohn of Lubavitch (1789-1866; known as the "Tzemach Tzeddek" after his Halachic work by that name), passed away on Nissan 13.

Links:
A short biography
More on the Tzemach Tzeddek

After reading the "Nasi" describing the gifts brought by the princes of the 12 Tribes of Israel for the dedication of the Tabernacle during the first 12 days of Nissan (see "Nasi of the Day" in Nissan 1), today we read the passage (the opening verses of the Torah portion of Bahaalotecha) describing G-d's instructions to Aaron regarding the lighting of the Menorah, which represents the contribution of the "13th tribe" of Levi.

Text of today's Nasi in Hebrew and English.

In preparation for the festival of Passover, in which even the possession of any form of leavened food ("chametz") is strictly forbidden, a final search is conducted after nightfall to remove every last crumb of chametz from our homes and property.

Click here for a step-by-step "Getting-Rid-of-Chametz" wizard. Click here for instructions on how to conduct the search, and here for the time for searching chametz in your location.

Links:
Sell your chametz online
About Leaven