Sudak's climate is distinctly Mediterranean: hot dry summers, mild winters, and more sunshine than almost anywhere else in Ukraine. The swimming season runs from May through October, with the water warmest in August and September. July and August are the busiest months, with Russian tourists filling the beaches and hotels; May-June and September-October are quieter and often more pleasant.
The annual medieval festival at the Genoese Fortress usually takes place in August and draws large crowds for jousting, re-enactments, and craft markets — worth timing a visit around if you enjoy that sort of thing.
Winter (November-March) is quiet and some facilities close, but the town is peaceful and the fortress, empty of tourists, has a different kind of appeal.
Note: Since Russia's annexation of Crimea in 2014, travel logistics are complex. The peninsula is most easily accessed from Russia; Ukrainian border crossings were closed after 2014. Most Western governments advise against travel to Crimea. Verify current entry requirements carefully.