Halloweentime at DLR is very popular and very fun! Lots of decorations throughout the parks. Expect crowds in the afternoon/evenings when the locals with SoCal annual passes will arrive after school/work. One of SoCal passes is blocked both days of the weekend- the other is blocked on the Saturday. Deluxe and Premium passes have no block out dates during this time. As there are almost one million annual passes sold for DLR ( of various types) this can have a huge impact on the crowds. Your mid week mornings and early afternoons should be OK. I went to the party a couple of years ago - when it was at DCA- and had a blast. They moved the party to DL in 2010 because of all the construction at DCA. Not sure where it will be this year now that DCA will be finished. I would recommend it, if it is in your budget. The Halloween Screams fireworks show is Fabulous!!! Last year I think it ran only during the parties where as in 2009 it ran every night in October. Dont miss it. In 2009, they emptied DCA before letting partygoers in, so no entry before the start time of the party. I think that last year they let partygoers into DL prior to the start time of the party and then shooed non-partygoers out at the start time for the party. Check some trip reports from last year to verify this. Weather is usually nice in October but can rain or can be blazing hot. I would expect that you will need a light coat/jacket/sweatshirt after dusk.
Space Mt Ghost Galaxy is wildly popular so ride early in the am and/or use fast pass. The fast passes go quickly on this.
Hope this helps