My sister has given birth to a boy, and we are wondering about the custom of what we've been told is called “Shalom Zachor”, which seems to be some type of home-welcoming for the baby held on Shabbat. What is the reason for this custom? Where is the ceremony held and how? Does it have anything to do with the brit milah, or is it something different?
Mazal tov on the birth of this new addition to your family! May you merit bringing him into the covenant of Avraham Avinu, and may you merit raising him to Torah, chupa and good deeds!
The term "shalom zachor" means peace upon/welcome to the male child. Thus the custom does seem to be based on welcoming the newborn into the home and community. It is indeed held on the night of the baby's first Shabbat. Family and friends gather to bless the child and mother, to say words of Torah and to make blessings over foods in their honor and merit.
It is often held in the house where the mother and baby are, but it may also be done in the shul, or really anywhere else that is convenient for a reception and is accessible to one's guests. The shalom zachor benefits the mother and baby even if they are not present.
The reasons for doing it on Shabbat is that this is a time when people are available and can visit with leisure and amity, and because of the beneficial influence and blessing of Shabbat itself on the baby. Indeed, the fact that the brit is performed no earlier than the eighth day after birth means that the baby will always undergo the uplifting experience of Shabbat before the brit.
That being said, it is not directly related to the brit, such that even if the brit is delayed for whatever reason, the shalom zachor is still held the first Shabbat. On the other hand, even if the shalom zachor was not performed the first Shabbat, it can still be done on another Shabbat.
It is very good to encourage people to prepare and say words of Torah in honor of the occasion, either on the weekly Torah portion or about the mitzvah of brit mila.
Customarily, one also offers a variety of cakes, fruits, nuts and seeds, and various beverages, including those used for a “L'chaim” for the guests to make blessings over in honor of the mother and baby.
Perhaps the most well-known food offered at a shalom zachor is cooked chickpeas or "arbes". There are different reasons suggested for this custom. Either because as a round food, it recalls the circular nature of life at this inception of the life-cycle; or, alternatively, its association as a food of mourners expresses our empathy with the infant's sorrow over parting from the joy of being taught Torah by an angel while in his mother's womb.
In either case, we bless the child that he embark upon a life full of Torah study and observance of the mitzvot.