If you have ever had a string of lights that stopped working after only a few days, it is likely because you were using them in an enclosed space. Using holiday lights in an enclosed space, such as hanging them inside a lantern or placing them under a glass cloche, can cause the bulbs to overheat and eventually fail prematurely.
When bulbs are used in an enclosed space, the heat generated by the light has nowhere to go and builds up around the bulb. This can cause the wire inside the bulb to break down and eventually stop working. The best way to avoid this is to use holiday lights in open spaces where the heat can dissipate quickly. If you must use them in an enclosed space, make sure there is plenty of ventilation so that heat can escape.