Question: What is special about 1961 Summer Universiade and Sofia, Bulgaria

Context paragraphs:
Volleyball at the 1961 Summer Universiade: Volleyball events were contested at the 1961 Summer Universiade in Sofia, Bulgaria.

Water polo at the 1961 Summer Universiade: Water polo events were contested at the 1961 Summer Universiade in Sofia, Bulgaria.

1961 Summer Universiade: The 1961 Summer Universiade, also known as the II Summer Universiade, was an international sporting event for university students that took place in Sofia, Bulgaria.

Athletics at the 1961 Summer Universiade: Athletics events were contested at the 1961 Summer Universiade in Sofia, Bulgaria.

Gymnastics at the 1961 Summer Universiade: The Gymnastics competitions in the 1961 Summer Universiade were held in Sofia, Bulgaria.

Basketball at the Summer Universiade: Basketball tournaments have been staged at the Universiade since 1959.  The men's tournament was introduced at the 1959 Summer Universiade and the women's tournament was introduced at the 1961 Summer Universiade.  The tournament was not held during the 1975 event.

Tennis at the 1961 Summer Universiade: Tennis events were contested at the 1961 Summer Universiade in Sofia, Bulgaria.

Sofia: Sofia ( or or ; Bulgarian: София , "Sofiya ", ] ) is the capital and largest city of Bulgaria.  1.26 million people live in the city and 1.68 million people live in its metropolitan area.  The city is at the foot of Vitosha Mountain in the western part of the country.  Being in the centre of the Balkan peninsula, it is midway between the Black Sea and the Adriatic Sea, and closest to the Aegean Sea.

Volleyball at the Summer Universiade: Volleyball tournaments have been staged at the Universiade since 1959.  The men's tournament was introduced at the 1959 Summer Universiade, while the women's tournament was introduced at the 1961 Summer Universiade.  After not being included in 1975 and 1989.

Diving at the 1961 Summer Universiade: The Diving competition in the 1961 Summer Universiade were held in Sofia, Bulgaria.

Supporting paragraphs: 1961 Summer Universiade Sofia
Answer: international sporting event