It's too bad
Legoland isn't open on Wednesday, because that would have been the best solution. I agree with the poster who suggests swapping your Legoland and SeaWorld days if you really want to pack in everything.
Here's what I would probably do though if it was me and I wanted to do all this stuff:
I would definitely absolutely end my trip with Disney, so I would go to Disney on Friday. I also wouldn't especially want to be driving on the freeway and getting exhausted by traffic and another theme park on my last Disney day, so I wouldn't choose to do either Legoland or SeaWorld on Friday.
I would probably do one of the following:
Keep Monday and Tuesday as Disney, move SeaWorld to Wednesday and Legoland to Thursday, and then do Disney again on Friday with or without the addition of the Halloween party.
Alternatively, depending on your arrival time and other planned activities, you could do a partial Disney day on arrival day Sun. Sept. 25, still do Disney on Monday and Tuesday, SeaWorld Wednesday and Legoland on Thursday or do Disney on Monday and Wednesday, SeaWorld on Tuesday, and Legoland on Thursday. Then on Friday, have a leisurely day. Sleep in, use the pool, go to downtown Disney, find another local attraction such as the Cube Museum in Santa Ana. Then do the Halloween party that night. It starts at 7 PM, so you can enter
Disneyland at 4 PM and stay until midnight.
Btw, I don't feel there is any hard and fast crowd rule at Disney. I have seen Saturdays that were less crowded than Wednesdays and of course vice versa. I hear that Wednesday is the least crowded comment a lot, but I also hear that off-season is less crowded too. If you look at the pictures of people in the parks last week and compare to some recent off-season pictures, I think there is room for doubt.
