Try as I may I can't find any XC▸ (Cross Country Trains (franchise)) Advance tickets available for Sunday 26 Nov or Sunday 19 Nov come to that. Yet strangely there are plenty available for the Sundays before and after. Even at the extremes of their network and times when I wouldn't have thought demand was that high (e.g. Edinburgh - Newcastle from 0800hrs to 1000hrs on those two Sundays) there are no XC advance tickets available yet all the VTEC trains do have them. Yet on any other Sunday XC tickets are plentiful at these times.
XC tell me that they've simple sold out their quotas for these two dates - but on every one of their trains yet not for other weekends? Can't help feeling this seems statistically unlikely.
Can understand how engineering work might impact availability as ChrisB suggests. This might be true for Sunday 19 Nov when there are XC diversions as Bristol Parkway is closed all day for the additional platform works, but this isn't the case for most of Sunday 26 Nov. And why should this knock on throughout the XC network?
Can the ticketing experts on the Forum shed any more light on this? Can't see why demand should be so much higher on these two particular dates. Is it something to do with how the XC demand management model is set up?