Here is how I see it from my angle.
When – is a time related pronoun; just like [where] and [which] are pronouns in most usage. I think we can agree on that.
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"A day will come when you will no longer remember my face" can be interpreted this way:
[There will be a time] when you will no longer remember my face. If it's true, [when] is a pronoun.
I have just built a vacation home which is near Yosemite National Park
I still remember the time when my father took me to the park
The same cabin where we had a family picture taken 30 years ago still stands.
I am slowly pouring water in a cake mix, and I said, tell me [when]. This means, let me know at the moment you want me to stop pouring.
I know him from way back when. = Long time ago.
I changed his diapers when he was a baby. = [when] in this case appears to be a conjunction to me.
The label of the function changes with different context. That's my two cents...