Smolensk is best visited in summer — May through September — when the parks along the kremlin wall are in full leaf and the weather is warm enough for comfortable outdoor exploration. July and August are the warmest months (around 20-22°C), and the long summer evenings make it pleasant to walk the wall and the Dnieper embankment.
Winter is cold (January averages around -5°C to -8°C) and snowy, but the kremlin wall and towers covered in snow can be beautiful. The historical sights are open year-round. Spring (April-May) sees the lilacs and lindens that line many streets come into bloom. There are no major festivals that make one season significantly better than another for visitors, but summer is by far the most comfortable.