This is a neat question to me. Because my very first trip to Disneyland (California for that matter) was in October for Halloween. We went the very first week. I remember our first day there and having this exact conversation, and it was brought up a few more times that week. It was like 6:00-6:15pm and it was dark. My mom and I looked at each other and said "When did that happen?". There was like no dusk or gradual sundown. To us it happened so quickly that it was like someone had shut the lights off. We said this many times during the week. Maybe we were too busy to notice it, but even one night sitting in front of the castle eating a corn dog watching the world go by, we said it was fast. We now go in May/June every year, and find that the sun setting happens a little later, and is slightly more gradual, but it's still fairly quick.
According to Calendar-365.com, the sun will setting at 6:06 on October 25. It will start to get dark around 5:30 in some areas of DLR dcue to tall buildings and such.