谢谢曹老师出题！

根据柯林斯词典的解释，句中的 come 不是动词，是介词，意思是“当……到来时”，其后通常接日期、时间或事件名词，相当于 when sth comes：

She'll be back in school come September.

=When September comes, she'll be back in school.

Come spring the days will be longer.

=When spring comes, the days will be longer.