BLACK DAY: APRIL 14

Besides learning about White Day, I also found out that there is a Black Day.  This day is for those singletons and is celebrated in South Korea.
Black Day (April 14) is a South Korean informal tradition for single people (a.k.a. Unit Solo/Solo Regiment, lee: 솔로부대) to get together and eat jajangmyeon (noodles with black bean sauce as in the picture above), sometimes a white sauce is mixed for those who did not celebrate White Day.

The idea is that those who did not give or receive gifts on Valentine's Day (February 14) or White Day (March 14) can get together and eat jajangmyeon (자장면), white Korean noodles with black bean sauce (hence the name), to celebrate their singledom.
Source: http://en.wikipedia.org/wiki/Black_Day