A fast day must be acceptable to G‑d. The moral lesson of a fast day will apply even when Mashiach comes. Then they will be days of rejoicing. The tenth of Teves recalls the siege on Yerushalayim, the first calamity which led to the destruction. But it also carries a positive aspect which will continue into the future. In the galus we must find the power to convert darkness to light. Yaakov lived his best years in Egypt. We are in galus in order to reach a loftier state. Tzedakah will reveal the higher inner power of the galus and bring the redemption.