Which pasuk in the Torah is the cornerstone for our entire avodas Hashem? Chazal respond in a medrash that it is the pasuk in Parshas Pinchas that mandates the daily offering, and encompasses the service of Hashem in its entirety: "Es hakeves echad taaseh baboker, vees hakeves hasheini taaseh bein haarbayim," (Bamidbar 28:4). What is so significant about the daily sacrifice that warrants its distinction as the foundation of our avodas Hashem?
There are two different challenges in avodas Hashem. Special occasions, such as yomim noraim, and shalosh regalim arise which obligate us to reach new heights of spirituality. There is, however, a second aspect of avodas Hashem and this is the daily avodah. It is relatively simple to reach spiritual heights on sporadic occasions. On a daily basis, without a specific excitement of the moment, it is much more difficult to attain such levels. It is this latter aspect of avodas Hashem that is symbolized by the korban tamid. It is neither the korban Pesach nor the avodas Yom Hakippurim that is singled out as the cornerstone of our avodah. Rather, emphasis is placed on our ongoing commitment every morning and evening.
It is the significance of the korban tamid that has linked this mitzvah to many tragedies in our history. One of the events commemorated and mourned on shivaasar beTamuz is the cessation of this korban. In contradistinction, we do not commemorate the termination of the korban Pesach or the avodas Yom Hakippurim.
Chazal relate to us another tragedy associated with the korban tamid (Bava Kama 82b). During the internal wars of the Chashmonaim, even though Yerushalayim was under siege, the korban tamid continued to be offered. The lambs for the korbanos were lifted over the walls of Yerushalayim in a basket. Eventually the Chashmonaim on the outside decided to stop this procedure, and trick their fellow Jews inside the walls by placing a pig inside the basket.
This was not merely a trick by warring factions within the Jewish people, but also a sign from Hashem regarding the deteriorated spiritual state of the Jews. A pig is the only non-kosher animal that has split hooves but does not chew its cud. It appears kosher, yet upon examining its inner being we realize that it is not. The Jewish people continued their external service of the korban tamid, but their inner-selves had become disqualified. Their offering korbanos while fighting amongst themselves was symbolized by the pig.
It is during the period of the three weeks, which begin with the cessation of the korban tamid, that we are required to rededicate ourselves to daily, pure avodas Hashem.