Since the previews for A Good Man Goes to War have promised that the Doctor will learn “who River Song really is”, this may be my last chance to go on the record with some wild speculation about what the reveal will be.
Let’s start with the most popular fan theory: River Song is the Timehead. Certainly, this theory fits the available evidence well enough, and nothing directly contradicts it. It would add a selfish element to River’s admonition to Amy that killing the Timehead would create a paradox.
There are only two problems with the theory. The first is that one of the pieces of evidence for the theory indirectly contradicts it. When the Doctor says that he suspects the Timehead “will find them”, he is looking at River. Proponents of the theory say this is a pointed, knowing look. The problem is, if he already knows River is the Timehead, then “the day the Doctor finds out who she is” has already come. Granted, this is a somewhat weak complaint. River may not know that he knows, after all.
The other problem with this theory, in the words of Phil from the TARDIS Eruditorum, is that Steven Moffat is cleverer than that. It is a somewhat obvious resolution from a man whose resolutions tend to be more surprising than that. So, at any rate, it would be somewhat disappointing if that were the extent of the reveal.
Phil’s favored theory (and the only other theory that feels plausible to me) is that River Song is simply River Song, a particularly amazing woman the Doctor falls in love with. This would make the preview a clever bit of misdirection and hype-building, and while it would probably disappoint many fans, I think it would be a satisfying solution. And this leads me to what is, to me, a more interesting question than who River Song is: what is the context of her statement?
Is River going to directly tell the Doctor who she is, or are we dealing with a scene she has foreknowledge of? “This is the day…” scans (in the context of Doctor Who) like something a character might say if they had travelled back in time and were observing their own past. Showing such an encounter to, say, Amy and Rory would be a nice way to tell us more of River’s story without waiting years for the Doctor to get there on his own.
Of course, this leads to another question: why go out of the way (both diegetically and from an audience perspective) to show us such a scene if there isn’t something radically important about River’s identity? This takes us back to the River-as-Timehead theory, which seems to have a whole lot of circumstantial evidence supporting it.
I’m not going to say that I actually buy in to the theory, but it is a somewhat interesting one. At any rate, it is a short wait; we’ll find out tomorrow whether either of these theories are correct, or whether Moffat has yet another truly surprising reveal in store for us.
Edit: After I wrote this, I noticed Steven Moffat said the following on twitter:
yes, you will find out who River is tomorrow. Thing is though - was that what you were REALLY asking?
And, well, no, it isn’t what we were asking. What we have really been asking is who did River kill, and why? So, River being the Timehead isn’t a terribly interesting answer to any of that, and the only reason anyone started framing the question as “Who is River Song?” was that the trailer for Series 6 included Alex Kingston delivering the line “This is the day he finds out who I am”. So, the real question is “will we find out who River killed?” Of course, the ‘he’ could feasibly not even be the Doctor. Intentionally misleading us is, after all, Moffat’s modus operandi.