If I understand your question correctly, you are asking whether you should do MNSSHP on Sunday or Tuesday so that you can do Epcot the day you're not at MK for the party.
In that case, I would go to the party on Sunday largely because Epcot is far more crowded on the weekend days. Then, on Tuesday, you will have evening EMH at Epcot.
One thing to know is that the Festival booths close at 9:00 regardless. However, I would advise you concentrate on the booths before they close and use the EMH hours for attractions.