Lublin is pleasant from late spring through early autumn. July and August are warm (22–25°C typically) and the old town cafes are at their liveliest. The Lublin Carnival of Flavours food festival runs in late summer. Winter is cold (often below freezing) but the city is attractive in snow, and the indoor museums and the Majdanek memorial are worth visiting year-round.
The city sees far fewer foreign visitors than Kraków or Warsaw, which is part of its appeal — you can wander the old town without crowds even in high summer.