You can visit Sofia at almost any time of the year, and you will experience the city in a completely different way, depending on the season.
In summer, the weather is hot and sunny, and the city relaxes, with people taking it easy in the parks, or escaping altogether to the slopes of Mount Vitosha nearby, while in the cold depths of winter, the mountain attracts even more people with the promise of some of the best skiing in the country, and festive markets in the city itself.
Spring and autumn are the best times to visit Sofia, as the weather is most pleasant, and close to the city there is plenty going on in the form of festivals and events. Also, away from the busier times, such as bank holidays, there are many cheap deals available on package holidays to the city.