Map from Google - Sofia

The climate of Sofia, capital of Bulgaria, is continental, with cold winters and warm summers. The relatively low latitude (which is the same as central Italy) is offset by the position in the Balkan Peninsula (which is exposed to cold waves from Russia) as well as by altitude: the city is located at 550/600 metres (1,800/2,000 feet) above sea level.
Here are the average temperatures.
Average temperatures - Sofia
SofiaJanFebMarAprMayJunJulAugSepOctNovDec
SofiaJFMAMJJASOND
Min (°C)-5-305101214141162-3
Max (°C)24915202426262317104
Min (°F)232732415054575752433627
Max (°F)363948596875797973635039

Precipitation is not abundant, since it amounts to 600 millimetres (23.5 in) per year, but it is well distributed over the seasons, with a relative maximum in May and June, because of afternoon thunderstorms, and a relative minimum in winter.
Here is the average precipitation.
Average precipitation - Sofia
SofiaJanFebMarAprMayJunJulAugSepOctNovDecYear
SofiaJFMAMJJASONDY
Prec. (mm)353540508080605040405545610
Prec. (in)1.41.41.623.13.12.421.61.62.21.824
Days77810121087668998

Winter, from December to February, is cold: the temperature remains below freezing (0 °C or 32 °F) even during the day for long periods. In periods of stable weather, fog often forms. Snowfalls are frequent, but they are typically light, though occasionally they can be more abundant. However, snow covers the ground for long periods.
During outbreaks of cold air masses from Russia, the temperatures can drop by several degrees. Here are the lowest temperatures recorded in cold periods over the last decades: January 1985: -26 °C (-15 °F); February 1991: -23 °C (-9 °F); January 1993: -24 °C (-11 °F); March 1987: -14 °C (7 °F); December 1988: -20 °C (-4 °F); December 2001: -20 °C (-4 °F); February 2006: -25 °C (-13 °F); January 2012: -22 °C (-8 °F); February 2012: -21 °C (-6 °F).
On the other hand, when the south wind blows, which sometimes can be strong, the temperature can rise by several degrees, so that there can be mild days, up to about 15 °C (59 °F), even in winter: the air comes down from the nearby Mount Vitosha, with a down-slope, föhn-like effect. In the warm winter of 2014, the temperature reached 18 °C (64 °F) in January, and 22 °C (72 °F) in February.

Spring, from March to May, is unstable, especially in March and April: the first mild days alternate with the return of cold weather, with possible snow in March, and sometimes also in April. In May the first warm days occur, with highs close to 30 °C (86 °F), and also the first afternoon thunderstorms.

Summer, from June to August, is warm and sunny, usually with cool nights, and some afternoon thunderstorms. Generally, daytime temperatures are pleasant, around 25 °C (77 °F), but there can also be heat waves, which typically do not last long, with highs about 36/38 °C (97/100 °F); however, air humidity is not high and nights remain cool. The temperature reached 41 °C (106 °F) in July 2007, and 39 °C (102 °F) in August 1994.

Autumn, from September to November, is mild and pleasant in September, and often also in the first half of October, then quickly becomes more cold, gray, and even relatively rainy. In October, at night it gets cold; usually the first snowfalls occur in November, when fog becomes quite common as well.

The amount of sunshine in Sofia is good from May to September, when the sun often shines, while it is low from November to February, when the weather is often dull and sometimes foggy.

Sofia

When to go

The best time to visit Sofia goes from mid-May to late September, keeping in mind that sometimes in July and August, and more rarely in the second half of June, it can be hot, so those who don't like the heat can avoid these periods. You should also take into account some afternoon thunderstorms, besides for the evening it's better to bring a sweatshirt, and maybe even a jacket if you go in May and September.