Bosnia has a sharply seasonal climate. The interior — Sarajevo, Mostar, the Bosnian hills — sees hot dry summers (25–30°C) and cold snowy winters that can drop well below freezing. The Herzegovina south, around Mostar and Trebinje, is closer to Mediterranean: hotter in summer, milder in winter.
Spring (April–June) is the best all-purpose season. Snow has cleared from the highland routes, the rivers are full from melt, and the wildflowers are out across the karst. Temperatures are mild and crowds are still light.
Autumn (September–October) is the other strong season. Warm dry days, harvest festivals, and the brutal summer heat in Herzegovina has eased.
Summer (July–August) is fine for the south but can be uncomfortably hot in Mostar. Worth timing a visit around the Sarajevo Film Festival in mid-August or the annual Mostar Bridge Diving Competition in late July.
Winter (December–March) is for skiing — Jahorina and Bjelašnica, the 1984 Olympic venues, are excellent and inexpensive. Bring warm clothes everywhere, including Sarajevo.