Detailed Instructions: You are given a concept, and a list of answers. You should generate a question about the concept that leads to the given answer(s).
Q: concept: Boston Celtics  answers:  ['1969 NBA Finals', '1986 NBA Finals', '2008 NBA Finals', '1974 NBA Finals', '1984 NBA Finals', '1965 NBA Finals', '1966 NBA Finals', '1976 NBA Finals', '1981 NBA Finals', '1968 NBA Finals']
A:
when did the celtics win championship?