One of my students had to fill in the gap (or leave it blank if the relative pronoun can be omitted) in this sentence:
My landlady says we can't use her sitting-room because we made too much mess the last time ________ she let us have a party.
He wrote "which". The answer key left it blank.
"Which" is not an option, while a blank or "that" or "when" are possible options. But I am having a hard time explaining to him why "when" can be omitted since grammar books seem to claim you can only leave out the relative pronouns "who/which/that".
Please correct me if you find any statements above are wrong.
And my question is: what makes it possible for "when" to be omitted? If possible, quote source for information.