The move to autumn has often been attributed as a large reason for the recent drop in ratings, but the close proximity of the series finale to Christmas actually lends itself to creative opportunities that previously didn't exist. When we had spring series of the show or very short runs that concluded in September or early October, the festive instalment existed in isolation - a standalone slice of Who that, by necessity, couldn't be connected too much to the events of the previous series.
The Christmas Specials have dabbled in exploring the fallout of the series finale before (see The Runaway Bride's Rose mourning), but Last Christmas is the first festive special to really feel connected intrinsically to the previous season, which was fresh in everyone's minds having concluded just a month before. In short, it's the emotional coda that Series 8 needed - Death in Heaven wrapped up with the Doctor and Clara abruptly separating, but Last Christmas manages to right that wrong by providing closure to their emotional journeys throughout the series.
For Clara, that means saying goodbye to Danny Pink. Danny's always been a divisive figure among fandom, but closure was necessary here before he could truly be forgotten (note how infrequently he's mentioned in Series 9). Last Christmas delivers the poignant, bittersweet moment where Clara lets go of Danny that needed to happen, closing off that part of Clara's life in appropriately definitive style while linking that goodbye into the ongoing conflict against the Dream Crabs. It's a superbly acted, emotionally charged scene between Samuel Anderson and Jenna Coleman that delivers the affecting idea that 'every Christmas is last Christmas' - and furthermore, it's a scene that proves the importance of this episode. Last Christmas vitally lays the groundwork for the more reckless, less grounded Clara who's lost almost all her personal connections to Earth and replaces it with increasingly risky adventures, setting up Series 9 while acting as an emotionally rewarding instalment in of itself.
This emotional closure and set-up is augmented by the fact that it's wrapped around a surprisingly solid story. Festive specials typically have a thin plot on which the usual trappings and plenty of sentimentality are hung - they're not episodes that often play quite as well when removed from the cosy, warm surroundings of Christmas. Moffat's specials have generally done a good job of providing a compelling plot on top of all the Christmassy trappings, and Last Christmas represents a festive special with enough intrigue and plot twists to work just as well as a regular episode.
The plot cribs ideas from all over the map, but it's nonetheless a clever and twisty one that mixes typical Moffat concepts, such as a monster that preys on a particular part of human nature (in this case, dreaming), with classic base-under-siege staples thrown in. The dream layers provide a constant air of unpredictability and clever surprises to proceedings, and though the 'this is a dream' twists are piled on to the point of absurdity by the end, there's still a neat thrill when Moffat pulls back to reveal his latest deception. Most of all, though, it's a plot that dares to go beyond the usual expectations of a Christmas Special as a sentimental bit of fun and little else - showing that it's possible to craft a purely exciting standalone episode that's not a major milestone for the show without having to lean on Christmassy trappings as a crutch.
And then there's the ending, which is a bit of a point of contention. This is a rare moment in the show where the gears are showing, so to speak - Clara's original exit before Jenna Coleman changed her mind is entirely intact, just with a new final scene stuck onto it that allows Clara's adventures to continue. The result is a muddled final scene that creates a poignant parallel between the older, weaker Clara and the Doctor, who himself was in a similar situation the Christmas before in The Time of the Doctor, then scuppers it with yet another dream reveal, heading in another direction that's joyful and exciting, but can't help but undercut what we saw before. It's not really the fault of anyone, but it's an example of real life dictating the plot - and in this case, there's only a flimsy bit of Christmas sentimentality to paper over the cracks.
Despite that unavoidable slump, Last Christmas is a great Special - packed with fun and enjoyment as every festive special needs to be, but with a rock-solid emotional core and an engaging, if not hugely creative plot. It's not a vintage instalment, but among the Christmas Specials the show has served up thus far, this is certainly one of the best.
And tomorrow, we get a whole new slice of festive Doctor Who as River Song returns in The Husbands of River Song!