The following list of national (and international) events that celebrate nature is surely incomplete. Please inform me of events that should be added (email: darwinsbulldog AT gmail DOT com, or comment below).
2012 | Year of the Bat
2012 | Year of the Lizard
2014 | Year of the Salamander
January | Winter Trails Day (1/12 in 2013)
February | Great Backyard Bird Count (2/15-2/18 in 2013)
March | National Day of Unplugging
March | National Walk in the Woods Day
March | National Wildlife Week
March 21 | International Day of Forests
April | National Environmental Education Week
April | National Park Week
April | Let’s G.O.! (Get Outside)
April | Save the Frogs Day (4/27 in 2013)
April | Earth Day
April | Arbor Day
May | American Wetlands Month
May | National Wildflower Week
May | Kids to Parks Day
May | International Migratory Bird Day
June | America’s Great Outdoors Month
June | National Get Outdoors Day
June | National Trails Day
June | National Pollinator Week
June | Great American Backyard Campout
June | World Forum International Mud Day
July 1 | National Admire A Frog or Toad Day
July | World Listening Day (7/18 in 2012)
July | National Moth Week (7/23-29 in 2012)
September | Serve Outside September
September | International Rock Flipping Day
September | Great Outdoors America Week
September 24-30 | Take a Child Outside Week
September | National Public Lands Day (9/29 in 2012)
October 5 | National Visit a Park Day
October | Earth Science Week (10/12-10/18 in 2014)
October | National Wildlife Refuge Week (10/13-10/19 in 2014)
October | National Fossil Day (10/17 in 2012)
December 5 | World Soil Day
exploreportlandnature.wordpress.com 
Today was National Admire a Frog or Toad Day. This is year is also designated the Year of the Lizard by Partners of Amphibians and Reptile Conservation.
Thank you Sara!