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 Ramerupt, 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.
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.
On Isru Chag, It is customary to hold a kinus Torah, a public gathering in which Torah thoughts are shared and discussed.
Thoughts can give life. Thoughts can kill.
For example, a soldier. The Torah commands a soldier at war, “Do not allow your heart to weaken. Do not fear. Do not panic.”
Imagine this: An army is racing towards you. Your eyes have witnessed their savagery. How can you command your heart not to fear?
Maimonides explains: Don't think those thoughts and you will not fear. Dodge them as you dodge a flying arrow.
Because such thoughts threaten not only your life, they endanger the entire nation.
A soldier must think only one thought: What must I do right now?
Today, we are all soldiers.