Guys, can anyone explain what does "D-Day" mean in Korean slang? I've heard it in so many different contexts that I have no idea anymore. At first I thought it was the day of one's 'first date' with someone, but different contexts have proved me wrong. So, what exactly is it? (I've also noticed all kinds of numbers being written after the "D-", like "D-50, for example, and etc.) Thank you for help😅

See a translation