When is for a time. Where is for a place. Do not swap them. The year when scores rose. The lab where they ran the trial. Relative pronouns as a set are in relative pronouns.
Time or map
A year, a decade, a morning, a moment: when. A town, a classroom, a graph, a table on a page is still not a time, so where only if it is actually a location. The century where the theory appeared is wrong. The campus when she studied is also wrong.
Places that are not quite locations often take in which. People take who, not which. Needed versus extra information is in which versus that.
The bottom line
Clock or calendar: when. Physical place: where.
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