Bad news to a client about a missed deadline

You are a paralegal at a small firm. The firm missed a non-jurisdictional discovery response deadline by two days because of a calendaring error. Opposing counsel has agreed to a short extension in writing, so there is no prejudice, but the client (a small-business owner named Marcus who is already anxious about costs) must be told. Write the email the supervising attorney will send. Under 200 words. Must: state plainly what happened and whose error it was, state the consequence accurately (none, with the reason), state the fix to the process, and say the client will not be billed for time related to the error. No minimizing words such as "slight", "minor", "just".