Hi Roger:
Congratulations on this upcoming special anniversary.
There does not seem to be any truly off season at Disney World, but as a general rule, crowds tend to be lighter when school is in session. So in my opinion, October is the perfect month to visit since schools are in session and crowds tend to be slightly lower than normal. You may notice some extra visitors for
Mickey's Not So Scary Halloween Party and
Epcot's International Food & Wine Festival (particularly in World Showcase), but no crowd should be too overwhelming. The crowds may not be so light that you will be able to walk on the most popular attractions, but you should see noticeably shorter wait times than you did during your summer visits. Weather in October generally hovers in the mid-to-high 70's, and hurricane season, which starts in the summer, tends to taper off by October. I would still be prepared for the occasional dip in temperature or a rain shower or two, but it should be a comfortable visit.
Since it's special anniversary trip, I would plan a special evening or two, such as a reservation at a signature restaurant, a
Characters in Flight balloon ride in Downtown Disney, or a
specialty cruise.
Enjoy your vacation.