Exterior lights and decorations should only be plugged into GFCI protected receptacles. A qualified electrician can install GFCI outlets or breakers as needed. GFCI (ground faulty circuit interrupters) help prevent electrocution or shock in case electricity comes in contact with water.
Never overload receptacles or circuits.. the rule of thumb is no more than 3 sets of incandescent string-lights per receptacle. Newer LED string lights draw less power and can likely have more than 3 sets plugged in together. The light packaging will tell you what type of lighting you have.