After leaving Morocco and before settling in Egypt, Maimonides visited Jerusalem and prayed at the site of the Holy Temple. Three days later, on 9 MarCheshvan, he visited Hebron and prayed at the Cave of Machpelah. Maimonides resolved to keep these two days—6 and 9 MarCheshvan—as a personal holiday (Charedim ch. 65 [5744 ed.).
Link: Rambam (Maimonides)
A Jew entered the Alter Rebbe’s study and said, “Rebbe, I am a simple man. Please explain to me: What is this holiday of Shemini Atzeret?”
The Alter Rebbe held his head in his hands for a moment. Then he answered:
“For a long journey, you might make a concentrate of the food and drink you will need for the way. At every stop, you mix some of the powder with water, and you have enough for that day.”
“Shemini Atzeret contains all the joy for the entire year, but in highly concentrated form. Treasure every moment!”