'I will miss the Smiths and the farm house which/that I've loved dering the holiday'
My grammar book says we can't omit 'which' or 'that' in this sentence eventhough it is an objective relative pronoun. Why is that?
I am not a teacher.
Your book is sticking to formal grammar, where the relative pronoun would be better left in. In colloquial speech many people would leave the pronoun out so long as the meaning is clear without it.