At the time of an event, contingent on that event happening at all.
1819, John Wilson, Cases in Chancery [1818-1819] page 90:
[T]o his son Thomas Battersbee, if and when he should attain the age of 21; but if he should die under that age then to his daughter Sophia, if and when she should attain that age...
1980, Louise Bates Ames, Frances Lillian Ilg, Your Three-year-old: Friend Or Enemy, page 81:
Since, as many parents have found to their sorrow, you cannot force a child to sleep, most find it easier on everybody to accept this nightwalking if and when it occurs
2006, Dennis D. Riley, Bryan E. Brophy-Baermann, Bureaucracy and the Policy Process: Keeping the Promises, page 318:
Still, one cannot completely ignore the differences in the nature of the paper trail the two sets of rules give us if and when we want to look back at a particular decision.