Unlike Christmas, Halloween is a bit newer to Japan and the reception to the the holiday is a bit more ambivalent. As studios like Trigger share their Trick or Treating art, some spell out that they're trying to have fun with the foreign costume festival tradition.
And from Shaft animator/designer Shigeto Koyama
And from their friends at Good Smile Company
From Shaft, with Theatrical Company Dog Curry, the folks who did Madoka's witch animation
☆♪Happy Halloween♪☆ 劇団イヌカレー（泥犬）さんから、ハロウィンほむらが届きました！ pic.twitter.com/P3PFDGybu3— シャフト (@shaft_official) October 31, 2015
Silverlink went with Non Non Biyori Repeat
Fate/stay night (and Zero) animators and their cafe