I'm still pretty early on in the phase of life where my friends reproduce a lot. I hope I'm not forgetting anyone, but I think this is only the second boy born in the past couple years to friends of mine who are active in Jewish communities. That makes this a seemingly appropriate time to talk about some weird quirks about brit milah, or bris, the Jewish ritual circumcision. (This probably won't sound as unusual to those of you not familiar with this stuff as did my posts on pidyon haben or siyyum bechorim or tchum or the 3 weeks, but it's presented in the same vein.)
A bris is held 8 days after a baby boy is born, health permitting. The day of the birth is counted as day 1, so it's usually held on the same weekday as the birth (keeping in mind that Jewish days start the sundown before). Traditionally, people are not invited to a bris. They're simply notified of the date, time, and location. It's understood that if you can go, you're welcome to. I believe the reason for this is that simply attending a bris is a mitzvah, a commandment, and you should never be put in a position of explicitly declining a mitzvah. (Sadly, this one is next Wednesday afternoon and not in Philly, so I won't be able to go.)
(This one courtesy of BZ.) A bris can be held on Shabbat or a major holiday, but only if it's exactly the 8th day. If you have to delay it for any reason, you should keep delaying it until after Shabbat or the holiday. One reason you might delay is if you're not sure which day a kid was born; if he's born during the 45 or so minutes of twilight between sunset and nightfall. That's sort of an ambiguous time in Judaism, sort of between two days. (For example, you observe Shabbat during that stretch on both Friday evening and Saturday evening.) If a boy is born during this time of day on Tuesday evening, you hold the bris on Wednesday rather than Tuesday the week later. And if a boy is born during this time of day on a Friday evening, you hold the bris a week later on....Sunday. Why? Because you can't do it on Friday, since the kid might've been born on Saturday, and doing the bris before 8 days is completely forbidden. And you can't do it on Saturday, because the kid might've been born on Friday, and you can only do a bris on Shabbat if it's exactly the 8th day. In an extreme example, if there's a two-day major holiday on Sunday and Monday, and the kid is born during twilight on a Friday evening, you hold the bris on...Tuesday! Of course. If the kid were born an hour later or earlier, you'd hold it on the 8th day as usual.
And, from one of my favorite books, this random quirk: Say that twin boys are born on a Monday afternoon in California, and then they are both taken to the Middle East, but one is taken the West route (i.e. over the Pacific Ocean and Asia) and the other is taken the East and more direct route (i.e. over the US, the Atlantic and Europe). Approximately 7 and a half 24-hour-periods later, it will be Monday afternoon in the Middle East. This is the appropriate time for the bris for the first boy, but the second boy will have to wait until Tuesday afternoon. The Date Line (which does exist in Jewish law, but may or may not be in exactly the same place as the International Date Line) does not apply to the bris, which has to wait until the 8th day of the boys life, counting sunsets and sunrises that he experiences in his location. Since the second boy saw one fewer sunset due to the direction he traveled around the world, he has to wait another day.