A small note I must add: I asked Amy Keating Rogers (via Twitter) where the Mane Six got their journal... ...and it turns out that this is the exact same book as the Royal Sisters' journal. The Mane Six just filled in all the empty pages. ...Well, you weren't using it anymore...
Well, I got The Journal of the Two Sisters... ...and I was disappointed to learn that there was ABSOLUTELY NOTHING in regards to where the Mane Six got this journal, nor if Celestia knows that they are using it. (Although that means that my headcanon that Celestia had this journal custom-made for the Mane Six still works.)
But since I had the McDonalds MLP Rainbow Power toys of Celestia, Luna, and Twilight...I could pretend that me, Twilight, and the Royal Sisters were all reading the Journal of Friendship together! ^_^
Doesn't it ANNOY you that we NEVER, EVER get to see what the Mane Six DO with their journal once they're done with an entry?
I mean, we almost ALWAYS got to see friendship reports being sent. But for SOME reason, the writers think it's a good idea to change the focus of the scene instead of showing what they DO with it. (I know that there's no time too, but as I said, since we almost always saw friendship reports being sent, it's awkward having them not answer the question of "What do the Mane Six DO with their journal after they're finished with an entry?")
I DO believe that the Mane Six let Celestia look at the journal entries, though. After all, in "Twilight Time", Twilight let Sweetie Belle write a journal entry, even though the journal rightfully belongs to the Mane Six. This vaguely hints that Twilight uses the journal to show Celestia what she's learned(on her own; after all, she isn't Celestia's student anymore), what her friends have learned, AND what Twilight has taught other ponies(Celestia DID want Twilight to be a teacher to others). AND, in the same episode, it's revealed that Twilight HAS been avidly researching the box, even though she's never been seen doing so onscreen. So it's equally possible that Twilight lets Celestia read the journal entries offscreen. Moreover, we NEVER, EVER see the Mane Six actually STARTING a journal entry...only FINISHING one. This opens up the possibility that the entries are still addressed to Celestia. (If you've read the book Dear Mr. Henshaw, you'll understand what I mean.) Alternatively, when the Mane Six have completed their journal(they DO appear to be past the halfway point already), they'll give it to Celestia for safekeeping. (The fact that the upcoming official book "The Journal of the Two Sisters" will be comprised of both the Royal Sisters' journal entries AND the Mane Six' journal entries seems to hint that the Mane Six will be all done with their journal at the end of this season.)
Still, considering how we almost always saw friendship reports being sent, it doesn't make sense why we don't get to see what the Mane Six do with their journal after thy're finished with an entry. It is DEFINITELY something we ABSOLUTELY MUST ask the writers.
(But there is one more thing that troubles me about the journal...WHERE did the Mane Six get it? I worry that their journal may actually be......cursed. ......Looks like I just thought up a new fanfic idea.)
In my head the journal is sent to Celestia and back after every entry / every other entry / every week or something like that. I think the reason we never see anypony start an entry because it would be boring to see them write "Dear journal" or similar every time. Giving the journal to Celestia for safekeeping makes no sense to me, Ponyville has a nice little library for storing books The idea of "finishing" the journal also makes no sense to me, it seems like a thick book to fill with such short entries so it would take years to acutally fill it. Even if they do Ithink they would probably just use a new one for the new stories. I can't imagine the journal to be cursed in the show but I would probably read a fanfiction about it
Sorry for double post, but...yeah, I agree that they'd send her the journal with the same regularity as the friendship reports. (In fact, I actually believe that the journal was custom-made for the Mane Six at Celestia's request. Explains why a journal would just so happen to have a cover illustration of the Elements of Harmony gems.)
And I have to note that the entries are actually fairly long; but the vast majority of an entry is omitted because the entry itself is primarily a recap of the episode it is written in.
And my "cursed" idea was more like this: "the journal is a prison for an evil entity, and with every entry written, the seal on the entity gets weaker".
...I'm actually hoping for an episode in Season 5 where the journal plays a key role in the episode's plot. A good idea is the "Trixie steals the journal so she can copy all its entries" thing that I mentioned.
I can imagine the journal playing a role in either the season 4 finale or a season 5 episode. I think it could vanish mysteriously and Twilight / Spike freak out the whole episode only to find it under the carpet in the end
...the journal DID play a role in the Season 4 finale; by going over the entries, Twilight realized where the keys were. THAT was the journal's true purpose in Season 4's story arc: telling the Mane Six how to open the box.
So you could say that, all this time, the Mane Six's journal was the book about the "mysterious" box that Twilight was looking for in S4E03. (But the question still remains...WHERE did Twilight GET this journal? (Judging by how it has a cover illustration of the Element of Harmony gems...I'm guessing Celestia had this custom-made for the Mane Six.))
But seeing as how the journal has now fulfilled its REAL purpose(promoting the book "The Journal of the Two Sisters"), I think that the Mane Six will go back to writing friendship reports in Season 5. (This makes sense when you consider the fact that, at a recent convention, Nicole Oliver(Celestia's voice actress) said that even she misses the friendship reports.)