The 13th Reality

I'm reading a book called The 13 Reality. It's so good. But I'm trying to figure out a riddle. Could anyone figure this out:


Mark your calendar. One week from the day before the day after the yesterday that comes three weeks before six months from six from now minus forty-nine days plus five tomorrows and a next week, it will happen. A day that would very well change the course of your life as you know it.
 I must say, I hope to see you there. 


If you figure it out comment what you think it is. But Tick keeps getting these weird letters from "M.G" It's so weird.