It’s February 29th, leap day. 2016 is a leap year, so this day is added to February. This to keep the calendar synced with the movements of the solar system, which inconveniently does not complete an astronomical year in an even number of days.
Leap year occurs almost every 4 years. For more details than you ever wanted to know, here’s the Wikipedia entry.
I’m surprised Hallmark does not have a card for leap day. Here’s what I found searching their site for leap day: http://shop.hallmark.com/search/?q=leap%20day