Celebrating Death

The Shulchan Aruch (Orach Chaim siman 580) lists numerous days of the calendar upon which righteous individuals passed away, and rules that one should fast on these days, since the death of the righteous represents a tragedy for the Jewish people.  Why, then, do many groups within contemporary Orthodoxy treat the "yahrtzeit" of the righteous as days of joy and celebration - particularly Lag Ba'omer?