Łódź is a year-round city but most pleasant in late spring and early autumn (May–June, September–October), when temperatures are comfortable for walking Piotrkowska Street and the parks. Summer can be hot and humid; winter is cold with occasional snow, which gives the old factory buildings a certain dramatic quality but makes exploring less comfortable.
The Light Move Festival in September is one of the best reasons to visit: an international festival of light art that transforms Piotrkowska Street and the city's industrial buildings into an outdoor gallery. It's one of the best urban light festivals in Europe and draws large crowds, so book accommodation well in advance.