There will likely be a LOT of Guests showing up early on Oct 1st at MK for several reasons.
The three I can think of immediately are -
1.- 'Vloggers' / Internet personalities showing up to provide 'coverage' of the day's events.
Anyone who's anyone within a 2,300 mile radius will likely be there.
Brace yourself for the onslaught...
2.- WDW merchandise buyers looking to get in line for any announced Limited Edition 'day of' 50th merch ( including local 'flippers" hoping to score some for reselling )
3. - Guests nervous that they might not be able to get into the Park later in the day, even though having a Park Reservation guarantees you admission regardless of the time you decide to enter.
The other draw is if Disney announces beforehand a 'opening ceremony' or other short staged acknowledgment of that day being the actual 50th Anniversary of the property.
If they announce in advance, this will also drive the uber-fans in droves to line up in advance hoping to get priority viewing spots.
Ditto for 'vloggers' who will be there in full capacity anyway.
-