What is significant about being eight days old from birth?

Question

We make a bris on the eighth day. A korban has to be eight days old before being brought on the mizbeach. The new moon according to many has to be eight days old from when its 'born' again till me make a brocho. What is the siginificance of the eighth day from birth, they must all have a similar reason. Although with a bris a reason is given that the mother who then becomes tahor can also join in, or becoming 'lmaalo', this cannot apply to the other cases, the moon is already 'lmaalo' at 'birth' and why should an animal become 'lmaalo'.There are many instances of 'eight' in the Jewish religion as other posts here have shown, and they could all have varied reasons. This question is pertaining specifically to eight days from birth.

Answer 1

There is a Midrash (Vayikra Rabba 27:10) that says:

דבר אחר: ‏והיה ז' ימים תחת אמו. ר' יהושע דסכנין בשם ר' לוי אמר: משל למלך, שנכנס למדינה וגזר ואמר: כל אכסנין שיש כאן לא יראו פני, עד שיראו פני המטרונא תחלה. כך אמר הקב"ה: לא תביאו לפני קרבן, עד שתעבור עליו שבת שאין ז' ימים בלא שבת ואין מילה בלא שבת, הדא הוא דכתיב: ומיום השמיני והלאה ירצה.

Without fully translating it, R' Levi is addressing why a Korban is not brought until the 8th day. He brings a parable of a king who decreed that before one may visit him, they must visit the queen. So too, G-d said that one should not bring a sacrifice until that animal has seen a Shabbat (Since after 7 days a Shabbat has definitely passed, no matter when the animal was born). So too, one does not perform a circumcision until the child has seen one Shabbat.

The Taz (Yoreh Deah 265:13) brings this Midrash as a support for the Seudat Mitzvah known as a Shalom Zachar.

The Chatam Sofer (Teshuva 102) quotes someone who connects this idea to waiting until after the 7th day from the Molad to say Kiddush Levana, although he questions it. See here for an analysis of the Teshuva.

See here, here and here for some other reasons why a Brit is done at 8 days.

Answer 2

Circumcisions and Offerings, the reason they need eight days is that both need all six days of the week as much as they need Shabbos. Otherwise, if all that was needed is Shabbos a boy born on Friday should be able to be circumcised after two days, and a korban brought.

And a רמז to this is found in that a sukkah is considered to be a דירת קבע on the basis of the word תשבו and a דירת עראי on the basis of the words שבעת ימים, which are joined together in the same verse בסוכות תשבו שבעת ימים. Therefore, the Torah is teaching us to elevate the concepts of temporary and permanent, along with their separateness, and to dwell with a greater understanding of Hashem’s Oneness. We must learn from a sukkah how to תשבו (dwell) in this world properly within an all-encompassing, elevated state of Shabbos, a Shabbos in essence that includes the six weekdays.

This is why Shabbos of the seventh day is called an אות, as the Torah states: ושמרו בני־ישראל את־השבת לעשות את־השבת לדרתם ברית עולם ביני ובין בני ישראל אות הוא לעלם כי־ששת ימים עשה יהוה את־השמים ואת־הארץ וביום השביעי שבת וינפש.

And a further רמז to this is that the word תשבו is a combination of שבת and ו (the six weekdays), which instructs us as to how we should dwell in this world with a perception of Hashem’s Oneness during all the days of the week, שבעת ימים. Therefore, the thirty-nine melochos of the Shabbos of the seventh day corresponds to the upper yud of the desert and is only meant to teach us that Hashem was, is, and always will be the source of every possibility, opportunity, power, and natural resource during the six weekdays. We must learn to see Nile River manna.

This is also why we find during the week of Yom Tov that Hashem arranged it that weekdays can be transformed into Shabbos with the experience of Shabbos Chol HaMoed and alluded to the unique nature of Hoshanah Rabbah as the Shabbos of the week of Sukkot, upon which melochos are permitted. This is not the Shabbos of the 39 melochos, but a representation of our achievement in recognizing a greater awareness of the Oneness of Hashem. This results in Shemini Atzeres, the eighth day that transcends the separateness of the six weekdays and Shabbos of the seventh day in Oneness.

And this is why the Torah in Vayikra 22:27 and Bereishis 21:4, teaches that a korban requires 7 complete days before it is acceptable as an offering on the eighth and upon which a male infant is allowed to be circumcised. Because, not only is the experience of Shabbos needed, it equally requires the light of each of the six weekdays. The two concepts need an integration into a unified understanding of the Oneness of Hashem. Only then can mature teshuva occur and the klipot of the six days of מצרים be separated from our hearts and minds and be called שאת. And this corresponds to Zos Chanukah.