A bit of a shower thought, but I found it curious that these two big afterlife celebrations from different cultures happen to fall on the same date. I'm aware of Halloween's Celtic origins, and Day of the Dead is Mexican, both holiday's are seemingly ancient, strange to think they evolved alongside each other in the late October/early November period and one didn't influence the other?
The imaginative side of me likes the spooky side of this coincidence, but surely there's historical reasoning?
So: Halloween is on October 31 because of its association with All Saints' Day on November 1 - and it was put on that date to coincide with pagan Celtic festivals around the same time, like you mentioned. Prior to Spanish colonization, Dia de los Muertos was a summer holiday, and it was moved to October 31 in order to coincide with All Saints' Day.