I'm guessing this is why Alex Kingston needed to cancel her appearance at the Salt Lake Comic Con this month.
The BBC has announced that Alex Kingston will reprise her role as River Song in the upcoming Doctor Who Christmas Special. Day one of filming the eleventh Doctor Who Christmas special will begin this week and is written by Lead Writer and Executive Producer, Steven Moffat, produced by Nikki Wilson and directed by Douglas Mackinnon.
Kingston commented on her return, saying "To be honest, I did not know whether River would ever return to the show, but here she is, back with the Doctor for the Christmas special. Steven Moffat is on glittering form, giving us an episode filled with humor and surprise guest castings. I met Peter for the first time at Monday’s read through, we had a laugh, and I am now excited and ready to start filming with him and the Doctor Who team. Christmas in September? Why not?"
Moffat added, "Another Christmas, another special for Doctor Who...and what could be more special than the return of Alex Kingston as Professor River Song? The last time the Doctor saw her she was a ghost. The first time he met her, she died. So how can he be seeing her again? As ever, with the most complicated relationship in the universe, it’s a matter of time..."
This will be the first time (at least, by our chronology) that River will meet Peter Capaldi's Twelfth Doctor, returning to the series since her last appearance in "The Name of the Doctor" in 2013.
River first appeared in the Tenth Doctor story "Silence in the Library/Forest of the Dead," also written by Moffat, encountering her in the 51st Century as a professor of archaeology who carries a TARDIS-color/pattern diary of her adventures and convinces the Doctor of his future trust in her by whispering his real name into his ear. River was later revealed to be Melody Pond, the daughter of the Eleventh Doctor's companions Amy Pond and Rory Williams. In "The Wedding of River Song," she married the Eleventh Doctor, then went to prison for his murder to corroborate a deception, though it is implied she regularly breaks out of prison to go on dates and adventures with the Doctor.
Doctor Who returns to BBC America for Series Nine on September 19, 2015.