I agree with DocV, but admit that it is the presence of the noun "afternoon" before the "when"-clause that made Sunshine think of the possibility of the clause being relative rather than adverbial.
DocV's sentence (2) clearly shows that both structures, "one afternoon" and the "when"-clause, are adverbial -- if the clause were relative, changing the order, that is, placing "one afternoon" after the clause, would not be possible.
Rather than syntax, I think it is semantics, i.e. the strong adverbial value of both structures, that makes us decide that the clause is adverbial. In these two sentences I found on the Internet the order cannot be changed, but a comma could be placed between "afternoon" and "when," which shows they are two independent adverbials:
- I started writing one afternoon(,) when I was twenty.
- One afternoon(,) when I was 9, my dad told me I'd be skipping school the next day.
In apparently similar constructions, with a time noun followed by a "when"-clause, the clause will be interpreted as relative if the time noun does not work as an adverb. In this case, the order cannot be changed AND no comma can be inserted after the noun (I found the sentences below on the Internet):
- I remember one afternoon when I felt extremely dejected and quite frankly frustrated that I didn't know how to move forward ...
- I remember vividly one afternoon when you called me into your office ...
Although "when" is more idiomatic, in both of the cases above it could be replaced with "on which."