Parshat Tazria begins with details regarding a woman giving birth and leads into the mitzvah of milah-circumcision on the eighth day.
The medrash says that the reason why milah is on the eighth day is to allow the baby to become strong (kedai sheyechazku kochotav). Common wisdom has it that milah is on the eighth day so that at least one Shabbat will have passed in every situation of milah. The Tosefet Bracha ties these 2 explanations together.
In Parshat Breishit the medrash presents the image of a tired, weak world following the creation of all things. Only after the seven days of creation were complete, when Shabbat came, were all creations strengthened.
Thus, the two ideas are one – after eight days the baby will have added strength because he has experienced Shabbat. Such is the nature of Shabbat since the beginning of time - it strengthens all of creation.
May we all be blessed with the invigoration and rejuvenation we so desperately long for this coming Shabbos!
Rabbi Neil Fleischmann