From The Century Handbook of Writing (1918) by Garland Greever and Easley S. Jones:
6. Do not use a when or where clause as a predicate noun. Do not define a word by saying it is a "when" or a "where." Define a noun by another noun, a verb by another verb, etc.
Wrong: The great event is when the train arrives.
Right: The great event is the arrival of the train.
Wrong: Immigration is where foreigners come into a country.
Right: Immigration is the entering of foreigners into a country.
Wrong: A simile is when one object is compared with another.
Right: A simile is a figure of speech in which one object is compared with another.