Officially it's 3pm, but you can use online check-in to let them know if you'll be there early and you can often get into your room before 3. It's not a guarantee but it can happen!
It's 4pm for deluxe resorts, when we stayed there we arrived at 7am and didn't get our room until 3pm, we got checked in and they text us when the room was ready, you can leave your bags with the desk to get your vacation started.
Standard check in is 3 PM but if you're there earlier they will usually let you check in if the room is ready. If not you can leave your luggage with bell service and go eat or to a park and they'll bring it to your room when it's ready.
We have had different WDW rooms ready for check-in at @10-11 am (but rarely so never count on it), just about official time, and everywhere in between. You should ask nicely if you really want to get in a room and it is hours before check in time. Once we got a different room simply by asking. Cast members really do want to help us and asking nicely goes a long way if you are asking for something special. That said, check-in is officially posted so I remind everybody NEVER count on getting in your room earlier.
Its 4pm. We checked in last week, one of our rooms was ready around 2. But the other wasn't ready until 4 on the dot. You can leave your bags with bell service and swim, or go to a park.
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