The number 7 is a significant number in the Harry Potter series...
7 Books
Ginny is the 7th Weasly
Born in the 7th century
Harry "Kills" Voldemort 7 times
7 horcruxes
7 years at hogwarts
you get the idea~

Well, 7 is the most powerful magical number...

"The one with the power to vanquish the Dark Lord approaches, when the SEVENTH month dies from parents who defied him thrice."

Now 7 books and 7 years at Hogwarts was already noted by Rowling when she was still writing the books, as there would be 7 books because 7 years is the duration of your training at Hogwarts, and thus one book for each school year (even though Harry didn't attend Hogwarts (at least not for lessons) in the final novel).

Now in many fantasy worlds either 7 or 8 have been of great significance when it comes to magic. In Terry Pratchett's Discword the number 8 is magical and wizards in the series even try to avoid that number as a result. In Harry Potter going for 7, due to the 7th month in which Rowling herself was born and all was the easier way to go, and using either 7 or 8 as a magic number is absolutely not new.

Now it was even stated literally that 7 was a magic number in the novels. Voldemort already asked Slughorn if it was possible to divide the soul in 7 pieces with Horcruxes based on 7 being magic, and I vaguely remember Hermione also stating 7 being magical in reference to her lessons in arithmancy.

Omg!!! That's so crazy!!! I knew both of those facts, I just didn't put two and two together. Wow!

Now maybe it sounds odd, but this is why I'll probably never forget the birthday of a woman I barely know... Well at least not personally. I have done the same thing by the way in my own work, as the main protagonist of my RPG Game "Star Story" (https://tricky.gamejolt.io/starstory/) Wendicka Lovejoy, was born June, 19th, 2975, making her exactly 1000 years younger than her spiritual father, however I only discovered Rowling did the same with Harry Potter, when the original plot for Star Story was already written, and I also have to add that Wendicka's love-interest has the same birthday as the wife I had when I wrote the final draft of the plot script, and I'm not sure if Rowling went so far. 😁

Of course, the birthday of the Weasley twins was also not randomly taken. They were born on April Fools Day... Does this tell you something about their behavior?

I think most of all it tells us about lazy writing... 😅 Most things in her books are fairly transparent, especially names. As soon as you look into the names of anyone who isn't a main character they will just be words for their occupation, describing their main physical or mental characteristic or just something having to do with their field.
Why is Remus Lupin named after a person who famously - at least according to legend - grew up with wolves and the latin world for wolf? When he was born he definitely wasn't a werewolf yet. Why is Luna named after the moon, especially because "lunatic" is the description most people would derogatorily describe her behavior with? I can almost forgive that one though, because it actually is a girl's name used in the UK... But Professor "Sprout" teaches herbology? Really? Is that all you can ever do when you were born into that family?
And the more obscure the character is, the less amount of effort did she put into naming them. Just look at the names she gave the authors of the different school books for example... 🙄 The more I look into it the less I like her because while she did create a pretty interesting world and somewhat of an engaging story to go with it she cut corners on sooooo many things... It gets more and more annoying every time. 🙈

Being a writer myself I can however nuance a few things. The birth date was clearly a way to identify herself with Harry, just as I did with Wendicka (who is to a certain extend a parody of myself). Now when it comes to Sprout being the herbology teacher, we can also relate to how surnames came to be, and that many people back in the day used the name of their profession, or a word related to their profession. Take the Dutch politician Frans Timmermans for example... "Timmerman" is Dutch for "Carpenter", so it's very likely his ancestor was a carpenter. And some families stay in the same profession for generations.
Sometimes it's also a kind of pun.... In my own work there is a young centaur girl named in Dutch "Ankie van het Grunnersven". If you know about about horse sports you can easily find out after whom she was named (although her personality was not based on her "real counterpart")... Come on anybody (even non-Dutch) should be easily able to riddle that one out :)
A nice one to know is when it comes to translations. Bellatrix Lestrange is in Dutch called "Bellatrix van Detta" as a clear pun to the word "vendetta", a name that suits Bellatrix well.

And indeed sometimes it just laziness such as "Septima Vector" as arithmancy teacher. And indeed Luna's name in order to make bad jokes about lunatics, was obvious, in order to fill in the school environment where bad jokes about names are quite common. Although I like the name "Luna". I already liked that name before Luna Lovegood was introduced to the series. Rhemus Lupin was indeed too obvious (and his pseudonym for Potterwatch.... Romulus.... Oh, damn).... :)

Well, I'm not saying you should never ever do this. I just can't take the books serious anymore but they clearly are meant to be serious. Yes, I know where surnames came from but it's just not how the world works anymore... Although the Wizarding World seems to be stuck in the past anyway... 🤔🤷 And yes, I absolutely love the name Luna as well. Always have.

In my own work I made a nice way around that. The Elfish tribe I write most about have a surname system where you don't get your surname because your parents were named that, but you have to earn your surname. The girl Cida Appeltaart for example... she earned her surname (literally meaning "Apple pie") because whenever she bakes apple pie it's always so delicious that even people who hate apple pie would die to have her bite of her apple pie. A girl whose first name is never spoken (she even hates it when you do) earned the surname "Flora" (and she also uses it as a first name) for her devotion for flowers and everything that grows. Call it cheating if you like... Of course since the wizarding world uses the same system we use, that trick didn't work 🤣

I actually quite like that system. One of the people in my "book" don't really have surnames, their last names are the name of the house they live in. So, at first you have the same name as your family but when you move out and build your own house you can name it as close or far from your parents' house as you want. It usually leads to people just expanding their ancestors' homes and very close family relations and few people really move somewhere new. And if you do build a new house it usually means something went wrong between you and your family and usually the people wouldn't even consider themselves related anymore. On the other hand it also means that guests take on the name of the place they're staying at and surnames and adresses are basically the same thing in those countries.

Ah yeah, house systems.... Another tribe (Befindo's a kind of winged humans) as a similar system of houses, although houses are there still more of families than actual buildings (and having a house is among Befindo's also a sign of nobility). Your system does sound interesting, though. I also have a tribe (Fairies) where you just have "son of" or "daughter of" and then the name of your father as surname. And the humans have surnames in a similar system we do. The Centaurs in my stories always have a surname with "van" and then the geographical name of the place where they were born (which could even be areas that don't belong to any Centaur tribe).

When it comes to the earn your name system. A character in my work (not a member of that tribe though) who is a criminal said about that system that it provides more identity than any other naming system. Since you have to earn your surname the surname tells people really something about who or what you are or what you do. It is a bit inspired by somebody once telling me that your nickname tells more about you than you given name.

Oh, I thought the "earning your name" was based on the Roman Empire. People would earn additional names by doing extraordinary things. Like, you might get the name Africanus if you've lead a successful expedition or war in Africa or you'd be called Caesar when you become the emperor. They still had both first names and family names, but you could earn many additional ones - the longer the name the more famous the person. I think George Martin stole a bit of that with all the names Daenerys accumulates over time.

I've never put that in my mind, which is strange if you imagine that I based the Befindo people when it comes to cultural expression much on Roman-Greek looks, not to mention that Yasathar, the supreme god in their religion, is loosely inspired by Zeus, so I did do some homework in those cultures for my work. I've never linked Daenerys' names to similar systems, but since nicknames are easily given to people in a position like hers. Of course, Gimli from Lord of the Rings would be given a name which means "Elfen Friend" in Dwarfish due to his friendship with Legolas. The earned names the Elves of the Drop-Out tribe use are however taken as an official surname even allowing them to use the earned name to sign official documents. Of course the system also fits the tribe when you realize the tribe's official motto is "Be yourself". I was however aware of it that I would not be the first to come up with an "earned name" system.

Y????

Why not?

Touche