i have attended UO on the columbus day weekends for the last five years.
i make it a 5 day event and stay on site.
Universal studios will close at 5 pm on the nights
HHN is going on.
IOA has closed at 6 in the past years.
later in the month of october, IOA hours can be extended some.
The halloween houses, stages, scream areas are covered in a way where you do not notice them in the daytime.
younger children at the park won't be afraid during the daytime.
Fear Factor will be closed during HHN as the stage is used for another attracton during HHN hours.
i stay on site and notice that most of those guests do not get to the park on the columbus day weekend until the afternoon. if they are there mainly for the HHN event, they rest in the morning for the night time afair.
the parks will start to get crowded by 3:00 -3:30 on the days HHN is operating.
you will be fine with the lesser crowds from opening time to that afternoon time.
on columbus day itself, i go to the park for the rides and shows.
i leave the park around 5 to get back to the hotel for my flight is later in the evening.
i do notice that crowds start on that monday usually after lunch time.
since i stay on site i do not have to deal with the ride lines but food counter areas do stay busy at that point.
hth
