7th or 8th day depending if you count inclusive or not.
Say conception takes place on Janury 1st.
Then Janury 8th is the 8th day counting inclusive. (Jewish way of counting)
Then Janury 8th is the 7th day counting exclusive.
Is that 7th/8th day of special Biblical significance?
Not 100% sure I understand your question. The Jews would count after the birth of the baby to the 8th day for circumcision and that's when the physical body spikes to the highest levels of vitamin-K that the person will ever ever have so that it's ideal for such a surgery in a primitive culture, and I'm thinking the immune system may also be in rare form on that day and then drop back down to normal levels for the rest of one's life, but I'm not 100% sure on the immuneology issue.
7th day from conception would probably theoretically be the baby's first Sabbath in this world, since the Bible sides with life beginning at conception as far as being in this world is concerned, and there may perhaps be something interesting in a Torah portion to follow up on anything insightful about the day of conception and the day of birth, if one wanted to get THAT DETAILED on studies, though I've never understood Jews to get that intricately involved with Torah portions though your much, much older, more studious and more reverential Rabbis might find it a fascinating thing to wanna keep track of for any additional insights that might come up about the life in question.
But how anyone could be 100% sure about the day of conception is beyond me, unless a marriage only involved sex once a year and on that particular day she happened to get pregnant. But even the most hormonally subdued wife might potentially murder someone in their sleep if that's all he could do. roflol Otherwise, one night stands, rapes, or a doctor artifically impregnating a woman would be other factors, but even then, I'm not 100% certain that anything of a regular sexual act would result in pregnancy within the hour or even within several hours of the sexual act. All kinds of factors are probably involved, and I think I saw somewhere when I was a kid that pregnancy could supposedly happen the following day or two from a sexual act, but I'm not sure if doctors and researchers still believe that, or if they're now siding with it happening within an hour or so. All of these kinds of factors would potentially mess with your math, if it did have special significance, so I'm bringing up the obvious about all of this.
If it were a matter of a desired pregnancy related to having a certain outcome, or which days have produced the heroes of history verses which days have produced the monsters or bums of history, that seems to be more a matter of the kinds of parents, the environment, the type of nurturing, the type of diet while pregnant, some genetics, and perhaps special blessings and other spiritual factors could be factored in, but I'm not sure if there's specifically magical days for when to become pregnant when it would be an extra special advantage, although each person knows their own body and can project 8 to 10 months into the future on whether or not the middle of summer or the worst part of winter is when they really want to be in that kind of position physically when they're suffering anyway if they don't do July or January well anyway on normal years because of temperature extremes.
But again, I'm not 100% sure if I'm understanding your question 'cause there's layers of the Bible that obviously speak to absolutely all levels of life and are relevant to each desired outcome, quality of life, et. al. So, if I went deeply weird on you, I could probably pick the brand of paper towels based on some Biblical precept, like a Universalist refusing to use anything other than "ALL" laundry detergent based on no other factor than the name of the detergent, not counting if they like how their clothes come out, it's agreement with their skin type, etc. There may be a deeply mystical tying together of truth that could be applied or a personal anniversary with Him that would be leaned towards on a pregnancy, but nothing where I could say chapter this of such and such a book, and this particular verse says "do these things" the week afterwards.
I've never heard of a specific Mosaic, Prophetic, or Psalmist statement about the week after conception. But as with anything else, if you wanted to really dig in the Bible for something, you could probably find some area of guidance or something personally special that He might speak to you or where it would click and you'd have peace, joy, contentment, rest, or ease about it, if you weren't particularly aware of actually hearing Him on any other level. If there were ever a way of being 100% certain of when that first week of new life were, then obviously it would come into that unborn child's first Sabbath and while the child likely won't take a 24hr break for religious observance on any cell divisions that it's doing, the day could probably still be used for speaking special blessings over the child, and the mother could likely get away with fasting and praying for a blessed child that lives a great life, free from tragedy, et. al.; prayer of Jabez type prayers could likely be prayed for the child and perhaps a trusted minister could be brought on the scene to participate in such spiritual activities. But I'm not aware of any specific Scripture specifically prescribing an offering 'til the child is born (which from the perspective of my prosperity theology just waved at me that I'm aware of no one that's either preached or practiced some version of that), and I'm not aware of any specific traditions of special Scriptural readings that were prescribed 'cause in ancient times, all a woman had to go on would have been skipped periods, morning sickness, and a growing stomach. There's nothing about the guy that'll let you know if he just fired a blank in bed or if that one was genuinely dangerous if she really really didn't want to get pregnant! roflol
Unless she just got sick IMMEDIATELY with morning sickness which let her know that PROBABLY she's pregnant, or unless she wanted to do something special with that 7th or 8th day later on her own "just in case" and to start marking it off with special time with Him every 7th day from then on, then that would be between that couple and God, then I neither know of how she could be sure apart from a prophetic Word or just having gotten sick immediately, as I said, nor am I aware of a specific Biblical commandment. One might simply choose to believe one got pregnant and start marking off that kid's Sabbath's, personally, [or that first one especially,] until she found out that she didn't get pregnant that time, but I genuinely don't see how one could ever really know in normal circumstances when that day would be apart from something intrusively done like a rape or a doctor's visit to get pregnant on purpose. Because married couples and even those that aren't even married but have chosen to go there premaritally will tend to be physically intimate more than one day at a time and usually several days in a row, to where there's no way of knowing if the first time or the dozenth time got her pregnant if she got pregnant.