Don't Open Till Christmas is absolutely abysmal but also an absolute must. Poor old Santa takes a right pounding in this one (speared, baked in hot coals, castrated in a London toilet etc). You won't see a worse film but you also won't see a more entertaining one.
The Film 2000 DVD (extras-free alas) restores all the (heavy) BBFC cuts but is slightly re-edited and misses out a Santa being shot. No matter. Grab it anyway.
Silent Night, Deadly Night urgently needs a UK release. Never been submitted over here, though the sequel was rejected back in 1987.
Other seasonal delights......
Silent Night, Bloody Night - Awful print and not particularly Christmassy but not a bad film nonetheless. John Carradine had a wonderful speaking voice but in this film plays a mute.
Pranks - Vaguely set at Xmas but devoid of atmosphere. Also devoid of 10 secs of a head-drilling because Vipco were too lazy to find an uncut print. An original Video Nasty but could well qualify for a '15' now.
To All A Goodnight - David Hess directed it and I have vague memories of a decapitated head in a shower and dead bodies buried in soil. Other than that it's been about 20 years since I last saw it and the memory fades a bit.
Tales From The Crypt - Ye olde superior British horror compendium classic which includes Joan Collins being pursued by a looney Santa after she murders her husband.
I'm sure I've forgot some but that's a taste of Xmas mayhem to consider.