Ptuj is worth visiting year-round, but two periods stand out. February brings Kurentovanje, the pre-Lenten carnival, which is the most compelling reason to visit — book accommodation well in advance if you're coming for the festival. Summer (June to August) is warm and pleasant for exploring the old town and swimming at Lake Ptuj, though crowds are manageable by central European standards.
Autumn is excellent for the wine harvest — the surrounding Podravje hills are some of Slovenia's most productive wine country, and harvest festivals run through September and October. The wine cellars beneath the town can be visited year-round.