DD day 

The day a woman celebrates when her bra size is promoted to a DD cup size

Girl #1: when was your DD day?
Girl #2: when I was about 13 and a half

DD day by Anonymous brms July 14, 2015

DD-Day 

Double D Day: The day when you plan on seeing a girl's large bust.

The term DD-Day was first used on an episode of Malcom In The Middle. In this episode Malcom's older brother Reese tried to get a girl with very large breasts to take off her sweater and stop hiding her god given breast.

DD-Day by TORQUE February 16, 2005

DD-Day 

Abbreviation for Drop Dead-Day which is the final,last possible day to complete a project of any kind within an established deadline.

Sally knew Tuesday of next week was DD-Day for the submission of her English paper on "The Modern Day Relevance of Shakespeare".

DD-Day by sally in norton June 2, 2014