There is an old custom that after a baby girl is born, the parents host a celebratory kiddush reception, typically following Shabbat morning services.

Earliest Source

Some trace this back to a verse concerning the leader Ibtzan (identified in the Talmud as Boaz): “And he had 30 sons, and 30 daughters he sent abroad, and 30 daughters he brought in from abroad for his sons.”1 The Talmud relates that he made two feasts for each child, “one in the house of the father and one in the house of the father-in-law.”2

There are differences of opinion as to the nature and reason for these feasts. One early commentator, Rabbenu Gershom (c. 960–1040), explains that the first 60 of these feasts were held in honor of the children’s births, and the other 60 celebrated their weddings.3 Thus, as far back as the times of Boaz, there was a celebratory meal when a girl was born.4

Others cite a non-extant Midrash that makes mention of a shavua habat, “daughter week,” a celebration associated with the birth of a girl.5

Now let us address the reasons for this celebration.

Blessing the New Baby

The party gives an opportunity for friends and family to gather (ideally but not necessarily with a minyan6 ) and bless the baby girl to grow in the ways of the Torah and good deeds, and eventually build her own home, continuing the chain of her Jewish heritage.7

Thanksgiving Feast

Another reason given is that the celebration is a seudat hoda'ah (“thanksgiving feast”), thanking G‑d that the mother and daughter are healthy and well.8

Celebrating the New Soul

Some have the custom of waiting to name baby girls on Shabbat. Now, the soul fully enters the body when the Jewish name is given. Accordingly, some say that the kiddush that follows the naming welcomes the new soul into the world.9

However, it should be noted that many, including Chabad,10 have the custom of naming the baby at the earliest opportunity, even if it is not Shabbat,11 and the (apparently unrelated) kiddush follows days or even weeks later.

The element of having people bless the baby is significant. In fact, there are many stories of girls and women who were facing various challenges in their lives, and the Rebbe (as well as other righteous Torah scholars12 ) would ask the parents if they had ever made a kiddush celebrating their daughter's birth. In the event that they had not, they would be encouraged to do so then.

If this is the main reason, why is the celebration on Shabbat?

Some say simply because it is a convenient time for social gatherings.13 There may also be an additional element of linking the celebration with the holiness of Shabbat.