Rabbi Yaakov ben Meir Tam, known as the "Rabbenu Tam," was one of Rashi's illustrious grandsons. During the Second Crusade, on the second day of the holiday of Shavuot, the Crusaders entered his hometown of Rameru, and pillaged and massacred many Jews.

They broke into Rabbenu Tam's house, plundered all his wealth, and seriously wounded Rabbenu Tam. On the next day, the 8th of Sivan, Rabbenu Tam escaped Rameru and the clutches of the Crusaders.

Two years later he completed his famous treatise on Jewish ritual and ethics, Sefer Hayashar.

Links:
Rabbi Yaakov ben Meir, the Rabbenu Tam
The Crusades

The day following a festival is called Isru Chag ("tied to the festival"). Tachnun (confession of sins) and similar prayers are omitted through the 12th of Sivan.