MSEP the FP+ window was 15 minutes prior to the start of the parade. The reserved area was right in front of the castle, and I got in pretty close to the start of the window and was in the 2nd row, so apparently they opened the reserved area early. I thought it was worth it to use the FP for this as it really saved a ton of time for a spot, and I was able to sit the whole time while I watched the parade, which was great. You enter the reserved area from the Tomorrowland side of the center of the hub.
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