An obvious fact to consider when planning to visit Ljubljana, but not the most important one, is the weather. Ljubljana has quite pronounced seasons, so it can be hot in the summer and cold in the winter. But generally there are not a lot of days with extreme temperatures, say in Jul/Aug the average temperature is about 18°C and in Dec/Jan it is -1°C.
[Source: Climate Zone]
Precipitation is another thing to consider, but it shouldn’t be a major problem as only rarely there are more than a few days of rain / snow in a row. And rain can be beautiful too. Or snow, especially if you haven’t seen it in your life before. Probably the best time to visit Ljubljana would be end of May / June / early July. Not because of the pleasant temperatures, low chance of heavy rains and lots of sun. But because of the people. After the winter mental hibernation, lack of vitamin D and winter food, there is a tidal wave of euphoria that nobody can resist. You don’t have to call your friends to meet them, you just call them and randomly meet with people you know and let the stories develop by themselves. The bars by Ljubljanica river are full of people. And it is the season of not only street performers that come from all over Europe, it is the time of open air and street festivals: music, theatre, circus, kinds shows and all other imaginable street art. It is the time when citizens of Ljubljana are really happy to live in this beautiful city.