1. A day that is filled with so much cleavage and/or of such great quality, that the entire quality of the day is increased. Usually for guys only.
2. Day in which you see down the shirt of your crushes blouse for the first time and is so great that the quality of the day is increased. Again, usually for guys.
1. Joe: "Man, my day sucked!"
Steve: "Mine would have been shity, but I had a Cleavage-tastic Day! I saw down Jessica Alba's shirt!"
2. Joe: "Dude, I had a Cleavage-tastic Day!"
Steve: "Really!?! You saw down Cindi's shirt?"
Joe: "Yes I did Steve, Yes I did".