Choose your season.
Ljubljana has a continental climate influenced by its proximity to the Alps and by nearby Mediterranean air streams. This gives the city four distinct seasons with July and August being the warmest months and January the coldest.
Perhaps the nicest times to visit for most people would be spring/summer when in May and June you can expect fewer showers and temperatures up to the mid 20s. You may need a light jacket to walk around the city in the evenings. Personally, I prefer even warmer weather and am happy to exchange the discomfort of 30+ Celsius for the more definite vacation feeling of being in the south of Europe and far away from home.
As the autumn months approach, the Ljubljana climate gets a little more stormy and you’re reminded that Ljubljana can be one of the wettest European capitals. There’s lots of fog during this period and into winter; a situation exacerbated by the city’s location in a natural topographic bowl.
In the winter, you realise that the Alps are much closer than you thought and snowfall can be significant. Unlike back home however, the city keeps functioning perfectly well as the municipality and population are well prepared for this weather. Note that if you’re here by car between October and April, you’ll be expected to have correctly marked winter tyres fitted though to be honest, you wouldn’t want to make the journey without them. You’ll also need proper boots and a warm coat for sightseeing as the temperatures can hover around freezing in the day and drop way below at night. Cafes and restaurants are kept warm and hospitable throughout and the occasional schnapps may help too (though obviously, I’m not a trained doctor).