is that the reason (that) they had to leave his country? (=why)today is the day (that) many of you have been waiting for. (=which)we flew to london that we could go ice skating spontaneously. (=where)

문제집에 이렇게 that이 관계부사를 대신할 수 있다고 적혀있는데,왜 첫번째 문장만 that이 생략될 수 있는 거죠?? 그리고 두번째 문장에서 that은 when 대신에 쓰인거 아닌가요???고수님들의 답변 부탁드립니다!!ㅎㅎ