2)
The day (when) Ken doesn't have to go to school is only Sunday.
3)
I don't know the (reason) (why) my sister is so angry.
4) We had a typhoon last Sunday. That was (why) the game was canceled.
5) He studied very hard. That was (how) he got the perfect score on his test.
【解説】 1 ) 先行詞の The hotel が後ろの節で「場所を示す副詞」として働くため, wher
2) 先行詞の the day が後ろの節で 「時を示す副詞」として働くため, when を選ぶ。