Decade 

Decades is the plural of Decade. 

A period of time equal to 10 years.

Each Decade will contain at least 2 Leap Years and some may include 3 Leap Years depending upon which year the Decade starts with.

For example the Decade from 2001 thru 2010 will have leap years in 2004 and in 2008 but the Decade from 2004 thru 2013 will have leap years in 2004, 2008 and in 2012.  Thus the number of days in a Decade can vary slightly.