Mostar has a Mediterranean-influenced climate — the hottest place in Bosnia in summer, and milder than the rest of the country in winter. July and August are reliably hot, often above 35 °C, and the old town fills with day-trippers from the coast. Go early in the morning or after 6 pm to avoid both the heat and the crowds.
The shoulder seasons — late April to June, and September to October — are ideal: warm sunshine, manageable temperatures, the rivers green and full, and far fewer tour groups. The annual diving competition off the Stari Most takes place in late July and is worth catching if your timing works. Winters are mild but quiet; many old-town restaurants close or shorten hours from November to March.