It does seem to be a lot busier during October due to Halloween time. We went last year, around early to mid October and experienced two back to back "Full Capacity" days which is when the
Disneyland park reaches its maximum guest capacity limit and closes park admission to newly arriving guests. Absolutely not fun being stuck in that crowd!
And I just got back yesterday. Arrived Wed. Oct. 6th and the crowds were great, very low. Thursday still pretty good, Friday and little more busy, but Saturday and Sunday were both terrible! Very crowded, long longs, and HOT! We did attended the Halloween Party on Friday, candy lines were long but well worth the wait, we were so busy with Trick or Treating that we only went on the Pirates ride but the ride was practically empty, Splash Mountain seemed calm too. I'm assuming Space Mountain was busy but most likely not as busy as during regular park hours - reached 85 to 90 minutes on Saturday. A castmember also commented on the crowds and too agreed that the October month seems to be busier.
On Friday night, the night of the Halloween Event, we had been in the park earlier but went back to the hotel to get ready, returned around 7:30 after the park had closed to the general public and California Adventure which was scheduled to close later had a huge mob of kicked-out DL guests waiting to get in, can't begin to imagine how crowded DCA must get during Halloween nights!