Dear Ms Costa,
I am writing to apologise for not sending you my section of the quarterly report on time. I know that you had to complete it on your own over the weekend, and I feel terrible about it.
The truth is that I simply forgot. I thought the deadline was the following Monday, and I did not check the calendar properly. I should have confirmed the date with you, and I take full responsibility for the mistake.
I am very aware that you were put under pressure because of me, particularly as you also had your own client presentation to prepare. It was unfair of me to leave you in that position.
I would like to make it up to you. I will take over the final proofreading of the report this week, and I will cover your part of the next monthly meeting. I would also like to take you to lunch as a small thank-you.
Kind regards,
Marta Costello