I just got around to rewatching this episode and remembered this thread. I think the above is too strong a statement about what is a relatively mild mistake. It is clear the events take place over two days and the fact that Christmas Eve was on a Sunday in 1989 is probably relevant. Let's also not forget that in an earlier Christmas episode, "And A Dewhurst up a Fir Tree," Foggy laments the previous Christmas being ruined by the trio waiting until Christmas Eve to do their shopping, only for it to fall on a Sunday when all the shops were closed. So in "Summerwine Land" shops are closed that day.
So yes, the street vendor does say it's Christmas Eve and then Compo says it is the next night. But nobody else ever says it, specifically Pearl at the beginning when she goes out of her way to simply say there is one shopping day until Christmas.
There are a couple of ways this mistake could have developed. Perhaps the episode was originally written with the idea that day one was the 24th and day two Christmas. Then they realized the 24th was going to be on a Sunday that year and decided to make it the 23rd and 24th but failed to make the street vendor dialogue change in the script. Or maybe the move to change the days happened part way through filming and they'd already done the street vendor scene as that was clearly done on location.
Either way it hardly impacts the episode. In the many many times I've now seen it I'd never noticed the error and I suspect I'm not alone in that.
