According to Michael Swan “After words referring to manner, time or place, that is often used instead of in which, when or where. That can also be left out.

I didn’t like the way (that) she spoke to me
I’ll never forget the Sunday (that) you first arrived.
Do you know anywhere (that) I can get a drink? “

I have been trying to find an example in which the relative adverb When can’t be replaced or omitted but can’t find any (in Defining clauses). Would you be so kind to lend me a hand on this issue?

Thank you very much.