I started typing my excuse. “I’m sorry, I was tired the day after the race, I had to go to the City, I had to…” when I caught myself justifying my faults.
Following my own advice I settled for “I am sorry for the delay. I will be there today”
An apology laced with a justification loses credibility. “I’m sorry I didn’t finish the assignment” is stronger statement than “I couldn’t finish it because I had to help elect the Pope”.
It does not matter what the reason is, big, small, or in between, the outcome is the same; you didn’t do what you said you would.
Excuses and justifications do not take away the mistake. Not owning it only makes it more disappointing in the eyes of the person who was slighted by your negligence. Admit a mistake, admit to have been remiss. Admit to have been negligent. Admit it with integrity. Do not give excuses. Own your actions. And don’t promise not to do it again, because you know you will.