Target creature gets -X/-X until end of turn, where X is the number of cards in your graveyard.
The value of X is determined only as Festive Funeral begins to resolve. The target creature’s power and toughness won’t change if other cards are put into your graveyard later in the turn.
Festive Funeral is still on the stack while you determine the value of X. It doesn’t contribute to the value of X.