Was the establishment of All Saints' day at that specific time of year really a syncretistic adoption of a pagan feast? Is it actually possible to know?
You might find previous questions about Halloween/Samhain informational while you wait for a good answer to your question.
1196 It seems that many modern Christians are not fond of Halloween. One of my history teachers said that this is because it is the only major holiday that the church failed to "Christianize." Is there any truth to this? particularly /u/circuitloss's comments here and here.