The Halloween Update was released on November 1st 2011 after the original Halloween launch was delayed overnight. It is a major update, and includes a massive amount of new content.
- Added chapter 5 - Sheol (unlocked after 10 mom kills or through Demon room in Womb 2)
- Added 3+ new bosses including Satan
- Added 4 new enemies
- Added 20+ new items
- Added new unlockable character - Eve
- Added one new “FINAL” ending.
- Added 6+ new achievements
- Added 2 new music tracks
- Added updated collection viewer
- Added 4 new tarot cards (finishing the major arcana)
- There was originally just going to be the Pumkin Mask as an update but Edmund and Florian thought that was kind of lame so they made a big update with many additions instead and removed the holiday item. 
- According to Edmund an update of this size was also made to make up for the shaky Launch of The Binding of Isaac.