MB response to an honest mistake

For context, NK had a swimming class right after his soccer class that just completely slipped my mind because out of the 5 classes that had been scheduled, 3 were cancelled. I keep track of schedules for 2 kids and their schedules are pretty packed and can vary week to week at times. Most importantly though, I’ve been with this family for 4 years and have never completely forgotten about a class before. Thoughts?