Slovenia's climate varies from Alpine in the mountains to Mediterranean on the short Adriatic coast.
Summer (June-August) is the warmest time, with temperatures of 25-30C in the lowlands. This is peak season for Lake Bled, the Soca Valley, and the coast. July and August can be crowded at the main attractions, particularly Bled and Postojna Cave. The mountains are ideal for hiking.
Spring (April-May) and autumn (September-October) are the best times to visit for most travelers. The weather is mild, wildflowers bloom in spring, and the forests turn golden in autumn. Crowds are manageable, prices are lower, and the hiking is excellent. September is arguably the single best month -- warm days, cool evenings, and the grape and olive harvests beginning.
Winter (December-March) brings skiing in the Julian Alps, with resorts at Kranjska Gora, Vogel (above Lake Bohinj), and Krvavec near Ljubljana offering affordable slopes. Ljubljana's Christmas market is charming. The coast is mild but quiet. Mountain trails are largely inaccessible from December through April.
Shoulder seasons are the sweet spot. Late May to mid-June and September offer the best combination of weather, crowd levels, and value. The Soca Valley is particularly beautiful in early summer when the snowmelt fills the river.