Admit you forgot instead of inventing an excuse. People forgive forgetting. Lies stick around.

Saying you forgot costs you one small embarrassment. Inventing a reason costs you a permanent risk, because the story has to hold up against everything anybody else might mention, and most invented reasons are checkable within a week. People are remarkably relaxed about being forgotten; everybody forgets things and nobody takes it as a verdict on their worth. Being lied to is a different category entirely.

Use the plain sentence: I forgot, I am sorry, here is when I will do it. No elaborate traffic. And if you forget a lot, the fix is a calendar rather than better material.