I'm sure (that) many of you like me, come December, get cards that contain these folded up sheets of colored paper, on which is written what is known as the family holiday letter.

划线 部分为which的先行词。由于定语从句主语很长，后移到谓语动词之后，构成了完全倒装句。这个定语从句与主句主语有何关系？为什么会提出如此问题？

本句特殊的地方是come December这个时间状语从句。come December是古体形式，come为动词原型式虚拟语气，由于省略了连词when, 所以虚拟语气动词倒装于主语前。如果不省略连词则为：When December come, 注意come为动词原型。现代英语在时间状语从句已经不用虚拟语气，改用陈述语气，故现代英语为when December comes.