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By definition restrictive relative clause modifies the meaning of its head word (restricts its possible referent). In the sentence "Summer is the season when I'm happiest." how "the season" restricted by "when I'm happiest"?
I think that "the season" already restricted/defined by the "Summer" and "when I'm happiest" is an non-restrictive relative clause which adds supplementary information.
You can say "Summer is a season." to define the word "summer", but if you say
"Summer is the season."
it's a meaningless statement, because you have singled out the particular season "summer", but haven't said anything about it to justify using the definite article "the". Adding "when I'm happiest." adds meaning to the statement, justifying singling out that particular season. In this example, that is why it is a restrictive clause.
