I got it on June 03, 2011 with ID# DE-929891

Not only this is my first UNESCO card but it is also my first Maximum card

From Wikipedia:

In philately a maximum card (also known as a maxi-card) is a postcard with a postage stamp placed on the picture side of the card where the stamp and card match or are inconcordance. In most cases, the cancel is also related to the image on the front of the card and the stamp.

The collecting of maximum cards is known as maximaphily.

Thanks to postcrossing user,unicorn69. :)