No, Borovets cannot be called snow-sure at Christmas: at mid-mountain, 1,900 m, there was enough natural snow for the holidays in roughly half of the winters 1984/85–2013/14, and in the remaining years you had to ski on runs with snowmaking. For Bulgaria that is the best result: more snowy Christmases than 75% of the country's resorts, noticeably more than Bansko (11 of 30 at 1,800 m) and Pamporovo (8 of 30 at 1,700 m). But among the 690 European resorts on the site, Borovets beats only 35%, which means that in the Alps it would land in the bottom half of the table with that score. The reason is the southern latitude of the Rila mountains: an altitude that would give a full score in Tyrol gives half here. Snowmaking covers the holidays in all 30 winters, so there will be runs at Christmas, but the snowy forest around them is not promised. The figures describe the climate of 30 winters and model scenarios: climate, not a forecast. The resort itself publishes how much snow actually lies on the slopes in the week you need.
Second half of the period and the scenarios: less snow for the holidays
Split the 30 winters in two and you can see that snow over the holidays has become rarer. In 1984/85–1998/99, 10 of 15 Christmases were snowy; in 1999/2000–2013/14 only 6 of 15; and days with at least 30 cm per season now average 105 instead of 122. In total, 14 of 30 Christmases passed without enough natural snow; they are listed further up this page.
The EURO-CORDEX models look further ahead, and they have to be compared against their own model reference 1986–2005, not against real-world series. In the reference, Borovets at mid-mountain averaged 8.8 Christmas days out of 14 with natural snow of at least 30 cm. In 2041–2060 under medium emissions (RCP4.5) 5.9 days remain, under high emissions (RCP8.5) 5.1. With snowmaking, both scenarios give 13 and 13.3 days out of 14, but days with snow per season fall to 86 and 72 instead of 117. How the climate scenarios are built is explained in the methodology.
When to go and which runs to count on
Borovets' runs on OpenStreetMap range from 1,329 to 2,538 m (43 km, 14 lifts), and snow across that range differs enormously. At the top, 2,538 m, 27 of 30 Christmases were snowy in the winters 1984/85–2013/14, with 149 days with snow per season; at the bottom, 1,329 m, only 4 of 30 and 52 days. So over the holidays the lower slopes are usually not covered without the cannons, and beginners learning at the foot of the mountain will in all likelihood be on man-made snow at Christmas.
Continuous snow cover of at least 30 cm of natural snow settles by 25 December in a typical winter and holds until 20 April; that happened in all 30 winters of the period. With snowmaking, the season stretches from 3 December to 8 May. At mid-mountain the season brings 114 days with at least 30 cm of snow (159 with snowmaking) and 151 days with at least 5 cm. The conclusion is simple: for natural snow, pick the second half of January, February or March, and take Borovets at Christmas only if snowmaking runs down low and, most likely, good snow up top suit you. Among the Bulgarian resorts it remains the best option for the holidays.