GREE is lying to you. I mean, GREE has had that same message attached to their main route mission completion letter (which only appears if you had bought and are wearing the more expensive clothing set) since before LAST YEAR's White Day. They have not bothered to edit it, so it confuses eager newer players all the time.
White Day was on March 14, and GREE only begins holiday events before the day happens. Although the Japanese BMP GREE gets these events, GREE has been ignoring its foreign players for several months. It's not happening, so let's look out for a different future event~