Since non of us is perfect and on number of occasions when it is necessary to apologize for a mistake, error or oversight. The apology expression is a step towards smoothing ruffled feelings. Express regrests and assurance that such an incident will not happen again. The letter should convey a sense of sincerity and cooperation on your part.Helping sentences of apology
Sample letter -For missing friend's party by slip of mindDear...
I never thought I would let something as important to me as your birthday that slipped from my mind.
It shows that I must be getting old.
I really apologize for the same. My only consolation is that I will have many more opportunities in the future to get it right.