The single biggest reason to time a visit is the Sinjska Alka, held on the first Sunday in August. The town fills with riders, brass bands, and visitors from across Dalmatia, and rooms anywhere within an hour's drive are booked out weeks ahead.
Outside of Alka week, late spring and early autumn (May–June, September) are easiest: warm but not blazing, with the surrounding hills still green. Summer is hot and dry — the Dalmatian hinterland gets considerably warmer than the coast, with daytime highs regularly above 35°C. Winter is cold by Croatian standards; the surrounding peaks see snow, and the bura wind from the mountains can cut sharply.
The Feast of the Assumption (15 August) draws large numbers of pilgrims to Our Lady of Sinj.