I have recently been watching Doctor Who (after 2005 series). I have finished season 6 and the platform I am watching it on doesn't have the special, "The Doctor, The Widow, and the Wardrobe". Can I just skip the special if it is not important to the story arc in next series? No spoilers please as I'm watching it for the first time.
Yes, you can freely skip all the main storyline of this episode. It's a classic one-off Christmas special, involving characters that never appeared in any other episode. There's no recurring villains like Daleks or Cybermen, and no worldbuilding or scene-setting that's used later.
The only scene which does fit into a (sort of, to the extent there is one) overall series arc is the very last scene, after the main adventure with the one-off characters is over, when
the Doctor goes to visit Amy and Rory. This is the first time they meet after he apparently died in the Series 6 finale "The Wedding of River Song".
Even that's not really important - just a bit of emotion, no big reveals or character development - but it's only 2 minutes long and you can easily watch it on Youtube: