The first one is at Cal Poly Pomona on Oct 15 & 16.
The second one is at Descanso Gardens on the following weekend, Oct 22 & 23.
Then the weekend after that, right before Halloween is the one at Kids Space.
And then there's the LA County Fair which closes next weekend (Oct 2), which isn't strictly related to Halloween. But, it's fun to take the kids to see the farm animals and such.