Phoebe: I hate my regular clothes now! Yíknow? I look down and-and I know that this isnít gonna be the most special day of my life.
We can replace the "of my life" with "in my life", cann't we?
"of my life" would be the more common idiom.
"In" seems to draw attention to something internal ("in my body"), but yet it could be used if you spoke of "the happiest hour in my workday" for example. I do not have an explanation of why. Perhaps someone else will?