The moment
Olga ruled Kyivan Rus' as regent after her husband Igor was killed by his subjects while collecting tribute in 945. She became the first ruler of the Rus' to be baptised, and visited Constantinople, in 957 by the usual dating, where Emperor Constantine VII received her at court. Her son stayed pagan, but her grandson Vladimir brought the whole land into the church in 988.
People: Olga of Kyiv, Constantine VII