Conversation so far:
When did Winnie Madikizela-Mandela meet Nelson Mandela?
1957
What happened to Winnie Madikizela-Mandela next?
Winnie Madikizela-Mandela was 22 years old and standing at a bus stop in Soweto when Mandela first saw her and charmed her, securing a lunch date the following week.
Did Winnie Madikizela-Mandela start dating Nelson Mandela?
yes

Question: When did they marry?
Self-contained rewrite: When did Winnie Madikizela-Mandela marry Nelson Mandela?