The Rebbe extends blessings for a good and sweet year and emphasizes the importance of spreading the teachings of the Baal Shem Tov as preparation for the coming of Mashiach. He encloses a Rosh Hashanah booklet with teachings of the Previous Rebbe and includes a brief reflection on time and spiritual growth during the Ten Days of Repentance.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/tishrei/773