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Unedited
My honored teacher and father-in-law, the Rebbe [the Frierdiker Rebbe], says (in one of his sichot) in the name of the elder chassidim:
"Chai Elul is the day that brought and brings vitality [chayut] into Elul."
It needs to be understood:
In the month of Elul, there are various matters and different types of avodot (divine service): matters of joy, and also matters of the opposite of joy — such as the concept of "cheshbon hanefesh" (soul-accounting) that is associated with the month of Elul.
As is known, the month of Elul is the month of accounting for the entire year. Particularly from Chai Elul, as my honored teacher and father-in-law, the Rebbe, says — the personal accounting for all twelve months of the year is conducted during the twelve days from Chai Elul until Rosh Hashanah, "a day for a month, a day for a month."
This "cheshbon hanefesh" is connected with a movement that is the opposite of joy in two respects:
For example: avak lashon hara (the dust of slander) — about which it is stated in the Gemara that "everyone is affected by avak lashon hara." Through such things, he became distanced from the Holy One, blessed be He, which brings him to a feeling of great bitterness.