The lifeday concept is an effort to change the culture of celebrating birthdays posthumously in favor of a less prophetic sort of celebration; an end-of-life date, or lifeday. The day when a living contribution to the historical record ends.
For example: March 14th, 2016 is not Albert Einstein's 137th birthday; April 18, 2016 would be his 61st lifeday. On October 5th, 2016 you might celebrate Steve Job's 5th lifeday.
The difference here is significant in that it: a.) celebrates the sum of a personal/individual contribution in a more complete way, b.) shores up the disconnect of a generational gap by seemingly drawing their influence closer to the present, c.) and removes the connotation of a kind of destiny by celebrating that they were simply born at all.
Don't get me wrong, birthdays are great for biographies, but lifedays keep living folks closer to their histories... in a way. Birthdays are for the living; lifedays are for the not-living.