Tzaddikim say that from the fifteenth of Av, Heaven begins preparing the chairs for the judgment.
Many have the custom of wishing people a kesivah vechasimah tovah from the 15th of Av. The gematria of באב עשר חמשה is וחתימה כתיבה טובה.
It states (Koheles 12:1-2) לֹא ֶׁראֲש עַד ...בּוֹרְא ֶיךָ אֶת וּזְכֹר וְה ַכּוֹכ ָבִים ַוְה ַיָּרֵח וְה ָאוֹר ׁהַשֶּׁמֶש ׁ ַ ךְתֶחְש, "So, remember your Creator... before the sun, the light, the moon and the stars grow dark..." The Or HaMeir explains that ׁ ַ ךְתֶחְש has two words in it, כח תש, "weakened strength." הַשֶּׁמֶש ׁ ַ ךְתֶחְש implies when the sun begins to get weaker, and that is on the 15th of Av, as Chazal tell us that on the 15th of Av the days begin to become shorter and the sun becomes weaker. ַוְה ַיָּרֵח means the month of Elul, as it states in parashas Ki Seitzei (21:13), ּאִמָּה וְאֶת אָבִיהָ אֶת וּב ָכְתָה יָמ ִים יֶרַח, "She shall weep for her father and her mother for a full month..." and the Zohar states that this refers to Elul. Thus, the pasuk is urging people to remember Hashem and to do teshuvah before the fifteenth of Av and Elul arrive. They certainly shouldn't wait for the last minute to do teshuvah; instead, they should begin the process of teshuvah now.
