5703 - 1943, on his forty-first birthday, the Rebbe published Hayom Yom, a small book that contains a chasidic message, culled from the writings and teachings of the Previous Rebbe, for each day of the year. The Rebbe put this work together at the behest of the Previous Rebbe who, several weeks later, wrote, “G-d granted my son-in-law, Rabbi Menachem Mendel, the ability to build this magnificent chasidic palace of 383 rooms [the number of days in that Jewish year]...”
1. Igrot Kodesh Rebbe Rayatz vol. 7 page 231