Should I use a GFCI outlet for outdoor holiday lights?

Yes, you should use a GFCI outlet for outdoor holiday lights — it’s required by code and cuts power if there’s a ground fault. You also need an outdoor-rated extension cord marked with “W” or “WA” for heavier gauge and sealed plugs. Don’t daisy-chain cords; use one long cord rated for the wattage and keep connections off the ground.