The Rema writes that the custom is to make a meal or gathering on Shabbat night after a son is born, and adds that this is considered a seudat mitzva (SA YD 265:12). The Drisha states that the basis for this custom is that the infant is in mourning for the Torah that he forgot. The reference is to the gemara in Nida (30b) which states that the embryo learns the entire Torah in the womb, but that when he is born an angel causes him to forget his learning. (The notes to the "Tur HaShalem" state that Drisha's source is the glosses of the Maharshal on the Tur.)
In other words, this gathering is really a kind of condolence visit. For this reason it is customary to eat chickpeas, which are a mourners food. (See Bava Batra 16b.)
In 305:3, the Drisha extends this idea. He writes that at night the custom is to visit the baby; in the daytime, we visit the mother. These two visits correspond to the "zakhor" ("remember", cognate with "zakhar" meaning male) and "shamor" ("keep", corresponding to the female) aspects of Shabbat. (Perhaps we should designate this daytime visit "shalom shamor".)
Putting these two ideas together, we may say that the new infant is in mourning not only over the loss of the Torah which he learned, but also over the separation from the mother. We have explained in columns on mourning that our beloved relatives are really a part of ourselves; when a relative passes away, the mourner's very identity is undermined. The customs of mourning give expression to this loss and simultaneously help the mourner on the way to reconstructing the self. The condolence visit is meant to ensure that the reconstruction of the self takes place in the context of the community.
This same process applies to a birth. In this case, the baby is literally a part of the mother's body; naturally, after the separation the child has to develop as an independent human being. This process combines a one-week period of reconciliation to the loss followed by the period of reconstruction.
In this unique case, both "mourners" - mother and son - are still alive, and so we visit and "console" both of them. The fact that this condolence call takes place on Shabbat, when such visits are some- what discouraged (see Shabbat 12b) reminds us that this particular kind of mourning lacks the inherent sadness of ordinary mourning. Birth is a vital separation which ushers in a period of renewed growth for both mother and child.
What about the birth of a daughter? We explained a few weeks ago that the separation between mother and daughter is much more gradual, because the bond between them is more powerful. This is hinted at in the Torah by having a doubly long period of tum’a after the birth of a girl (Vayikra 12). The tuma is also a kind of mourn- ing period over the loss of potential life; this is one explanation for the fact that many kinds of tum’a are for a period of seven days. In this case the period is twice as long, because the separation takes place more gradually. Another law which expresses this idea is the fact that the parents say "shehecheyanu" separately on the birth of a girl, but "hatov v’hametiv" jointly on the birth of a son, as we wrote. The separation is not acute enough to justify a conventional one-week "mourning" period on the part of either mother or daughter.
This is directly related to the insight of the Drisha. Mother and son are differentiated by gender; one represents "zakhor" and the other "shamor". But mother and daughter are united in the more passive "shamor" aspect of Shabbat; the separation is both more gradual and less intense.
The Drisha (YD 264) adds that the mila takes place on the eighth day because this follows the week of mourning. After the week of mourning the infant has to go forward developing his own unique identity; this is when the mila is performed to perfect his bodily form and when he is given a name. This column is dedicated in honor of Barry and Rachel Lynn of Efrat and their new son Yair Shmuel, for whose Shalom Zakhar the drasha was prepared.