Are you talking about a morning or evening reception?
I assume evening. If that is the case, then you can being the reception with a Illuminations viewing, which would start at around 8:30PM or so, but for an Ill party, there are several costs involved. If you do not want to include Ill in your reception (many have it as thier pre-reception), then you could prolly start it sometime around 9:00PM.
If you are speaking of a morning ceremony/reception, then the above poster told you the correct times, you must be out of EPCOT by 10:30 AM, but I do not know how early you could begin anything there.