The Be’er Heitev wrote: ” When the month of Elul enters and a person writes a letter to his friend, he should indicate at the beginning that he requests for him a good year.” The Shevach HaMoadim writes that from the beginning of Elul until Yom Kippur, Jewish people have the custom of blessing each other – both in written form (at the beginning or end of the letter) and verbally, to be meritorious on the forthcoming days of judgment “To be written and sealed for a good and sweet year.” In the Dvar Malchut for Parshat Re’eh (Torat Menachem Hitva’aduyot 5751, P. 178-9): “And especially so (the end of) the month of Menachem-Av (when we bless the forthcoming month of Elul, and in which falls the first of the days of Rosh Chodesh Elul,) it is already connected to the months of Elul and Tishrei (which is the beginning of the winter season), as the custom is known that from the 15th of Av and onwards we bless our fellows with the blessing to be written and signed and sealed for a good year…” In note 33 there it says that the 15th of Av is mentioned because the numerical value of the Hebrew words “K’tiva v’Chatima Tova” (Be written and sealed for a good year) equal the numerical value of the Hebrew words “Chamisha Asar B’Av”, (the 15th of Av). (Darkei Chaim v’Shalom)
