1) A day will come when you will no longer remember my face.
2) Tell me when you will arrive.
Tense simplification is a non sequitur here.
This is used when from the main verb it is obvious what time the speaker is referring to.
A day will come when you (will) no longer remember my face.
tense simplification here means you can omit will in the sub clause of sentence 1, but not in your second sentence.