We look at the calendars for 2015 and 2016
2015 is a non-leap year and 2016 is a leap year.

           in         in
          2015       2016 
 This  was/will be  will be 
 date      on         on
-------------------------------------------
Jan 13     Tu         We
Feb 13     Fr         Sa 
Mar 13     Fr         Su
Apr 13     Mo         We
May 13     We         Fr
Jun 13     Sa         Mo
Jul 13     Mo         We
Aug 13     Th         Sa  
Sep 13     Su         Tu
Oct 13     Tu         Th
Nov 13     Fr         Su
Dec 13     Su         Tu

It's interesting there are 3 Friday 13's in 2015,
but there will be only 1 Friday the 13th in 2016. 

In 2015, the 13th was or will be on the same day of 
the week in the months: 

Jan & Oct.
Feb, Mar, & Nov,
Apr & Jul
Sep & Dec

In 2016, the 13th will be on the same day of 
the week in the months: 

Jan, Apr, & Jul
Feb & Aug
Mar & Nov
Sep & Dec

So when Feb 13 is on Friday in a non-leap year (as it was
in 2015), or Jan 13 is on Friday in a leap year, there will 
be 3 Friday 13ths that year.  Otherwise there will only be 2
or just 1 (as will be the case next year in 2016).

Edwin