guy fawkes night, which takes place a few days after halloween, celebrates a time 400 years ago when/during which the british houses of parliament were nearly blown up.

 

when은 이해가 가겠는데 왜 during which가 쓰이나요??

 

during은 그냥 전치사 아닌가요? during summer 이렇게

 

during은 그렇다 치고 which가 온게 이해가 안가요. 앞에 400 years ago를 which가 꾸며주는 건가요? 혹은 앞에 a time을?

 the british houses of parliament were nearly blown up during that time 문장 나눠쓰면 이렇게 되는건가요?