Each geocacher can earn Souvenirs by finding caches on specific days, as defined by Groundspeak. I’ve now collected a few, and thought I would gather the list here, in the order they were acquired, along with a link to the logs of caches which earned me them.

Earned on 09/13/2008
United Kingdom
Earned on 13/09/2008
First Cache in Spain
Earned on 30/06/2009
Earned on 10/10/2010
International Geocaching Day 2011 Earned on 20/08/2011
International Geocaching Day 2011
Earned on 20/08/2011
First cache in USA - Florida
USA Florida State
Earned on 26/01/2012
Leap Day 2012
Leap Day 2012
Earned on 29/02/2012
First cache in Japan
Earned on 02/09/2012
Get Outdoors '13
Get Outdoors ’13
Earned on 13/07/2013
1st August 2013
1st Aug 2013
Earned on 01/08/2013
7th Aug 2013
7th Aug 2013
Earned on 07/08/2013
26th Aug 2013
26th Aug 2013
Earned on 26/08/2013
The Explorer
The Explorer
Earned on 01/08/2014