Here's my question) Which verb can I use with UP to say that something will be soon? For example "Christmas's coming!" What can I put instead of "is coming"? Is it possible to use "keep up"?
You can say "Christmas is coming up" to mean that Christmas is approaching soon.
In this instance the apostrophe is incorrect in my view. If you were to say "Christmas's celebrations are coming up" that would be appropriate, as there is a possessive relationship (I'm not sure of the correct grammatical terminology, but I'm sure others can clarify/correct me on this) between Christmas and celebrations, i.e. we are referring to the celebrations of Christmas.
It is not appropriate to use "keep up" which has a different meaning, as in "Keep up the good work".