in my opinion i dont think it matters which day of the week you go. if u are going right around halloween i would want to go during the week unless you want to see all the halloween shows and things like that
DH and I are also going in Oct. for 1 day. We have extended our WDW trip by three days, so we will probably be at Universal on either Friday or Sunday. Since this will be during the Epcot Food and Wine Festival, I'm hoping that many of the locals would be in Epcot that weekend and not in Universal!
Weekdays are all practically the same. And it's not like any specific day has the same attendance, week after week. Anyone who tells you any specific weekday is better than the other is just trying to look like they know what they're talking about.
The only weekday that would be more crowded during Oct., would be Friday due to HHN.