"Halloween" seems to be a feminine noun in the dictionary I looked, therefore your translation would be correct...
However ; it sounds a little weird to me. I think one would more likely say "Joyeux Halloween"
Other may disagree ; but Halloween isn't a huge celebration in France... So we don't have a lot of material to make our minds.
Hello! I just wanted to ask a question about the pronunciation: In France, does "Halloween" (masculine, right?) start with a "h aspiré" or a "h muet"? (Should there be a liaison between "Joyeux" and "Halloween"?)Nous vous souhaitons un joyeux Halloween