Choose the season by the kind of trip you want rather than by a single idea of perfect weather.
Spring
Good for greener landscapes and fewer peak season crowds, but weather can change quickly and mountain conditions may still be wintry higher up.
Summer
Best for long days and broad activity choice. It is also the period when timing, parking and early starts matter most.
Autumn
Often excellent for photographers, walkers and travellers who prefer a quieter feel. Daylight shortens and some seasonal services reduce.
Winter
Choose winter for atmosphere rather than a guarantee of snow in Bled itself. Kranjska Gora and higher areas are better bets for a stronger winter landscape.
The useful answer
The best month depends on whether your priority is warm weather, hiking, photography, swimming, winter atmosphere or lower crowd levels.