An illumination system operable for use as a string of Christmas lights may include a plurality of lamps connected in series. Each lamp may include a lamp luminary, a lamp stem, a lamp holder, and a pair of cables connected to the lamp holder. The lamp stem may have an insertion portion. The lamp holder may have an accommodation space formed therein. When the lamp stem is disposed in the accommodation space, the insertion portion may be clamped between a pair of first protrusions to separate the protrusions. When the lamp stem is removed from the accommodation space, the pair of first protrusions may resume contact through the elastic nature of the cable and the protrusions. The illumination system may be simple in structure and may be convenient for use.