我个人理解：网友给出a summer 应该是指的在夏天的某种情况或者某个地方，而不是指夏天这个时间名词。
a summer where the sun never sets 这个名词短语可以看成是下面英语句子的省略形式。
(I'm in some situation/place in省略) a summer where the sun never sets
(I'm in省略） some situation/place in a summer where the sun never sets
= (I'm in省略）some situation/place where the sun never sets in a summer
= (I'm in省略）some situation/place where in a summer the sun never sets
= (I'm in省略）some situation/place in which in a summer the sun never sets
= (I'm in省略）some situation/place and in it in a summer the sun never sets
以上句子结构类似于下面句子结构
I’m above the Arctic Circle, where in summer the sun never sets.
=I’m above the Arctic Circle, above which in summer the sun never sets.
=I’m above the Arctic Circle and above it in summer the sun never sets.
备注：以上where in summer the sun never sets为非限制性定语从句，修饰名词the Arctic Circle,引导非限制性定语从句关系副词where=above which, which指代the Arctic Circle. where=above which在非限制性定语从句作地点状语。
以上仅代表个人理解，不一定正确，仅供网友参考。