תנן בסיום מסכת תענית לא היו ימים טובים לישראל וכו’ ה’תשמ”ב.

This Ma’amar which the Rebbe repeats rather often is about the idea of two levels of Yomim Tovim. The greater of the two types are Teshuva Yomim Tovim such as Yom Kipur (when we got the second Luchos) and Chamisha Asar BiAv (when the Beis HaMikdash is built after destruction). He speaks of three levels: 1) Tzadik, 2) Teshuva and 3) Complete Teshuva, which is what will be after Moshiach comes and is connected to the Yerida of 9 Av. He explains what after Teshuva Shlaima there can not possibly be another Yerida and Golus. The evil has been made into a merit.