Festive dress typically refers to articles of clothing with sparkle and/or texture (for example: sequins, velvet, etc.). You could wear a black skirt or pants and pair it with some sort of sparkly top. You could also wear a fun cocktail-type dress (knee length or a bit above the knee). You could even dress up a nice "little black dress" with sparkly shoes and/or sparkly clutch and some fun jewelry. This time of year, the stores are filled with festive holiday wear, so it shouldn't be difficult to put something together.