...it is a "holy day",

是一个定语从句

Indeed, whatever shopkeepers gain out of Christmas, it is still a "holy day" ,the words from which "holiday" is derived, and it gives people time to pause and coacentrate for moment on non-commercial values.

这里 the words 是 "holy day" 的同位语。在the words from which "holiday" is derived 中，the words 是先行词，which是引导词，结构为 "holiday" is derived from the words.

如果简单的句子的话就是：
The holy day is derived from the words "holiday".
定语从句，相当于
it is a "holy day“ from which "holiday" is derived 。

这里从“，”以后为定语从句，修饰的为“holy day”这个词，
意为：
“holy day”这个词是来源于“holiday”.

the words from which "holiday" is derived = the words which "holiday" is derived from.
这句话是说，“holiday"这个词起源于 "holy day"

是定语从句，the words from which“holiday”is derived是从句，做了“holy day”的定语，这句话也就是说“holy day”is derived from“holiday”。

楼上的说的很好