In Universe theory: something to do with being a royal from the Moon Kingdom. I wouldn't be surprised if Naoko had wanted Queen Serenity's bday to be June 30th, thus Kousagi would most likely have the same bday too, I think.

Out of Universe theory: I think along with what you posted (which was interesting!), it may also have to do with their horoscopes! The ruling planet/celestial body of Cancer is the Moon, so what better birth date to give Sailor Moon (and then her daughter) is one at the end of June. I'm not super knowledgeable with astrology so maybe some else could add on to that idea.

There may also be some connection with midsummer and the summer solstice but I'm not quite sure on what yet.

I have always said that Naoko was a brilliant astrologer.
I'm just going to leave this here...

Quote :

The last day of the month that carries a message from Venus, June 30th, has a special story to tell. Being connected to the planet of love, devotion, inspiration, and balance, it is the final peak of its energies and easily bumps into its dark side, reaching debts that go far beneath the surface. People who were born on this date feel the need to invest their hearts in something deeply meaningful and won’t settle for less than finding beauty in all truths that life carries their way.

Quote :

The Sabian symbol for Cancer representatives born on June 30th in two years preceding a leap year: "A Group of Rabbits Dressed in Human Clothes Walk as if on Parade" The Sabian symbol for Cancer representatives born on June 30th in a leap year and a year following it: "A Small Naked Girl Bends over a Pond Trying to Catch a Fish" Sabian symbols for this date obviously speak of certain roles that may seem connected with sexuality at first, but really speak of naked vulnerability. It is the rabbits and the girl that play the game of getting dressed up (or down) and they stand for those who are simply trying to survive the day roleplaying or hunting. The inevitability of pretending becomes obvious in our own gut feeling that comes to surface as we imagine this little girl and start asking ourselves why she is naked in the first place. The soul of those born on this date needs to become bare, easy to see, and show childish vulnerability in its core, so these individuals can finally find oneness with another human being.

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The love life of individuals born on June 30th usually revolves around sexuality and the depth of convictions they share with their partners, for years. They search for someone to truly connect with, someone to help them find a hidden meaning their life has, and they might choose partners who are highly spiritual or turned to belief systems that differ from their own. Religions will change throughout their bonds, and their convictions are to change, becoming a magical tool towards materialization of everything they truly need.

As they madly hold on to the sense of magic life offers every day, they should never give up on their chase for the ideal partner and the ideal love story found in their heart. As healers and curious, yet emotional, partners, they tend to stick to people who are wounded and bruised until they fix them and help them regenerate from hurt. Unfortunately, this often puts them in a position of a therapist or a helper, that doesn’t seem to find the love and support in other people when their dark moment hits them with a hammer.

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Seeing hope when others don’t, they feel alive and awake most of the time, even though circumstances around them are tough to handle and others would rather run away. They have something to teach and something to share.

Ooooh where did you find all this! I didn't think to just look up June 30th specifically when I was looking up stuff on Cancer.

On a side note: I found it interesting that Saturn is in detriment to Cancer. Meaning, it is positioned in the zodiac sign opposite the sign it rules and when it is in detriment it is said to be not comfortable in that sign. Brings a tie to the Saturn/Moon ying yang thing with their powers!

I wonder if this means that Mamoru and Usagi share an Anniversary day with Usagi's parents? (Or around that time period?)  Because it must be a very special time  in September, when Usagi/Chibiusa/Or Kousagi were conceived in order to be born in June.

(My fan theory is because Ami's birthday is in September things got carried away at her birthday party for Mamoru and Usagi. )


I think it'd be funny if Kousagi was born in August like Mamoru. xD;



I also think Cancer is the only horoscope to be ruled by the Moon just like Leo is the only one ruled by the Sun. Each of the girls are born in a month that's associated with their planets. (or rather the ruling planets.  Some have multiples, like Scorpio's got Mars and Pluto as a ruling planet but Aries only has Mars-as far as I'm aware.)

It also makes sense too, that both Usagi and Chibiusa's crystal colors are specifically Silver and Pink.  Silver- the color a lot of people associate with the moon and purification anyway (in some ciricle) and Pink- feminine (as of the 1920s-ish time period) and originally the color Naoko wanted to use for Usagi to being with too.  

So, it'd make sense for that. The colors probably bleed over into the horoscopes to a degree maybe.

Making Chibiusa have the same birthday I think is part of the gag that they're so alike. What better reason to have similar personalities when you have the same birthday? Y'know, if you're superstitious :p. Which is easy to be in a world of magic!

Of course Star's quotes make the connection even deeper. I had no idea about these Sabian things before!

Quote :

The Sabian symbol for Cancer representatives born on June 30th in two years preceding a leap year: "A Group of Rabbits Dressed in Human Clothes Walk as if on Parade" The Sabian symbol for Cancer representatives born on June 30th in a leap year and a year following it: "A Small Naked Girl Bends over a Pond Trying to Catch a Fish"

Indeed, if you calculate Usagi's birth year based on her age at the time the series first debuted, it is 1978 (source: moonprincess.com) - two years before a leap year. So the Sabian symbol works there. Since we have no idea what Chibiusa's birth year is due to the future being vaguely in the 30th century, there's no way to confirm if the same symbol is meant to apply or not. The vulnerable girl seems to me to be just as valid an option considering her stint as Black Lady. But I don't know what the fish connection would be... Tuxedo Unmasked estimates Chibiusa is born in the year 2000 in the manga - a leap year - making her a representative of the fish pond girl at least by his calculation. (But he also calculates Usagi's birth year as 1977 so... Maybe? XD)

As for Kousagi... Wikia says her birthday is the same but I'm not sure what the source is ^^;;. I just reread Parallel Sailor Moon from Miss Dream and it's not in there. If I had to choose, I wouldn't make it the same. That's just too much X'D. Or maybe that's the point. But I could definitely see her being another Cancer!

It's odd that some sources have Kousagi's birthday as June 30th, but assumed. 

Moonsisters has it stated as June 30th, 2005, as do many sources. It is not confirmed, but it apparently is what the fandom has decided. 
Considering it is Usagi's second child in the universe, it is safe to assume they have the Same birthday, as it is something to do with the Moon lineage.

Personally, to me, I prefer her birthday to be like Mamoru's.  But that's just me. It's mostly because there's no 'official' date and also she never goes by a 'moon' title either.  BUT!

There is one shot of her using an attack summoning rabbits that looks like she's using the crescent moon wand. 

Any way, I'd probably put her birthday a day or two before or after the 30th.  A gag is fine for Chibiusa and Usagi, but I think it's overboard with Kousagi.  <<; 

  (I don't go by moonsisters.org. For multiple reasons.) 

Neither do I, but every source had it listed lol. XD

lol -true-  I'm going bonkers trying to find this one picture of Kousagi that I'm like, certain Naoko did but I can't seem to find it anywhere.   (It appeared somewhere around the time that she did Usagi and Chibiusa's birthday picture with cake??? )

Now, I'm not 100 percent sure if it actually existed or not. xD;

I'm interested why you guys think Queen Serenity would have the same birthday as well. I hadn't thought about it at all before.

Oh...
But if she was born on the moon/in space (?????), wouldn't her calendar look a little different than the one we use here on Earth? XD

That is absolutely true.  My working theory is they function off of Saros Cycle.   (It's basically how they calculate when next  the sun and moon eclipse occurs. So once every eighteen years and some change.)

It's make way more sense if Q. Serenity functioned on that cycle, wouldn't it?  Then her birthday would be whenever, really. (Possibly the idea that the Moon Kingdom has even their own zodiac based on the star positions too but different creatures maybe.)

Plus, Saros would be super interesting if they age based on that cycle too.  If one Saro is 18yrs, 11 days and 8hours. 

lol My math skills are terrible, but I have this idea of what if Q. Serenity and everyone ages by Saro years.  1 Saro for each aging year or something after they reach an age of maturity?

(My wobbly math will only allow this much at 1am.  If Q. Serenity is  18 Saro Cycles , that means she's roughly  324yrs 198days and 144hrs old in earth terms)  xD; someone with better math skills pls help.

I only suggested Queen Serenity because I figured if Naoko had gone with the whole chain of Usagi and Chibiusa sharing bdays, why not QS? Also the whole cancer/moon thing. I also have the feeling Naoko didn't go as much into detail with QS as others characters so who knows.

It also would help to fit in with my headcanon that QS, then Usagi and Chibiusa as well are all incarnations of Cosmos (as the same with Chaos and its incarnations through the main bad guys in SM).

I'd like to add my little two cents into this. Along with the entire birth-date numerology, astrology and how it could help determine personalities-What if Naoko made it so that modern-day Usagi, the incarnation that readers and viewers would be most familiar with would have more with Chibiusa in common? I mean, Chibiusa is practically Usagi's younger sister! Sharing a birthday with a sibling would be fantastic, in my opinion; and would make their bond even stronger.

I've always thought it had to do with astrology. But perhaps it's linked to how moon people are conceived.

starshine wrote:

I've always thought it had to do with astrology. But perhaps it's linked to how moon people are conceived.

I'm in full agreement, Sailor Moon has lots of astrological references and the planets the senshi represents shows their fullest powers and strengths from the very planets.  

In the astrological ephemeris, Chibi should have an Aries moon - the first sign of the western zodiac.
https://www.astro.com/swisseph/ae/2000/ae_2005.pdf

And if Usagi is born in 1977, her moon sign is also Cancer, but if it's in 1978, she's a Taurus. 
https://www.astro.com/swisseph/ae/1900/ae_1977.pdf

I would think Sailor Moon/Usagi have an Aquarius nature, while Chibi has a Capricorn one (rising signs?), which would be fan fic material instead of the official Sailor Moon Universe information.

The sign Cancer rules the Moon.
Queen Serenity probably didn't have a Sun sign, being that she wasn't from Earth.