Parshas Tazria discusses one of the most well-known and observed mitzvos is that of bris mila (circumcision). It stresses that the Mila must take place on the eight day, and the Gemara learns out that this is the case even if the eight day falls on Shabbos one must perform the Mila even though it involves one of the Melachos (creative activites) that are usually prohibited on Shabbos. What is the significance of having the bris on the eight day in particular?
In order to answer this question it is instructive to analyze the significance of certain numbers in Jewish thought. The world was created in six days, and on the seventh day, HaShem 'rested', thereby creating the concept of Shabbos, the day that we refrain from physical creation and focus on more spiritual pursuits. Accordingly, the number 'six' symbolizes the physical world, whereas seven represents the infusion of spirituality into the physical world. On Shabbos we strive to elevate physicality through using the physical world leshem shamayim (for the sake of Heaven). Thus, there is an emphasis on eating good food, and dressing nicely, but not for selfish reasons, rather to use the physical world as a kli (tool) for connecting to HaShem. The number, 'eight' symbolizes spirituality that is beyond this world, going beyond the laws of nature. Removing part of our body represents elevating ourselves beyond our natural physical drives. Some commentaries write that one of the reasons for bris mila is that it weakens man's natural physical lusts (see Moreh Nevuchim, Rabbeinu Bachaye). In this vein, bris mila represents a Jew's disassociating himself from the regular laws of olam hazeh (this world), and clinging to a completely different level of existence.
The idea that bris mila represents transcending olam hazeh is seen in the Torah's account of HaShem's command to Avraham Avinu with regard to this mitzvo. HaShem tells Avraham, "walk before me and be complete." Rashi explains that Hashem was instructing Avraham to perform bris mila and thereby attain completion. Immediately after this, HaShem tells Avraham that He is changing his name, which up till that point, was Avram, to Avraham. HaShem was taking Avraham to a whole new level of existence, and bringing him out of the limits of mazal which had thus far prevented him from having children. It seems clear from the pessukim that this promise and the promise of an eternal bris between HaShem and Avraham's descendants were dependent upon Avraham making his own covenant with HaShem, that of bris mila. Thus, we see that bris mila is intrinsically connected to the fact that the Jewish people live on a whole different plane of existence.
Rav Dessler zt"l applies this explanation of the difference between '7' and '8' to clarify a difficult Yalkut. The Yalkut tells us: "Shabbos and mila argue with each other. Shabbos says, 'I am greater than you' and mila says, 'I am greater than you'... from the fact that mila overrides Shabbos , we know that mila is greater than Shabbos." Rav Dessler explains that there are two ways in which a person can go about his avodas HaShem. One is to be involved in the physical world and elevating it for the sake of Heaven. There are numerous mitzvos that fit this category, for example, giving tzedoko (charity) is a way of using one's money to connect to HaShem, and as we mentioned above, Shabbos is the primary example of elevating physicality. The second way of growing in spirituality is by removing oneself from physicality, and thereby separating from his natural taivas (desires). Mila represents this form of avodas Hashem.
Rav Dessler points out that there is a great danger in the first type of spirituality where one tries to elevate gashmius (physicality) in that a person can easily fall into the trap of thinking he is elevating the physical world, however, in truth, he is really being pulled after his physical desires and the yetser hara is tricking him into thinking that he is doing it leshem shamayim. The second form of spirituality of removing oneself from gashmius does not pose this threat because one avoids the risks of being trapped. Rav Dessler writes further, that the only way that a person can be sure that he can use the physical world in the correct way is by also somewhat removing himself from it for a time.
With this understanding, Rav Dessler explains the meaning of the Yalkut. Shabbos represents the form of avodas Hashem where one uses the gashmius for spiritual purposes, whereas mila represents serving HaShem by weakening one's attachment to the physical world. Mila 'overrides' Shabbos in that it avoids the risks of being trapped by the yetser hara into becoming overly attached to the phsyical world whilst performing seemingly spiritual activities.
We have seen that bris mila represents separation from the physical world as a way of becoming closer to HaShem, and how this form of avodas Hashem is essential to one's spiritual growth. In this vein, my Rebbe, Rav Yitzchak Berkovits shlita points out that despite the stress on being involved with the physical world for spiritual reasons, the main way of achieving greatness is through involvement in purely spiritual endeavors. The great Torah Sages did not become great through eating leshem shamayim all the time. They became great by developing an overriding interest in spirituality and a disinterest in the physical world. Numerous stories are told of how unimportant food to Gedolim such as the Chazon Ish zt"l and Rav Yechezkel Levenstein zt"l. May we merit to emulate them and learn from the mtizvo of bris mila to focus on learning, davenning, and growth, as the main ways of becoming great.