In July 2015, Eton accidentally sent emails to 400 prospective students, offering them conditional entrance to the school in September 2017. The email was intended for nine students, but an IT glitch caused the email to be sent to 400 additional families, who didn't necessarily have a place. In response, the school issued the following statement: "This error was discovered within minutes and each family was immediately contacted to notify them that it should be disregarded and to apologise. We take this type of incident very seriously indeed and so a thorough investigation, overseen by the headmaster Tony Little and led by the tutor for admissions, is being carried out to find out exactly what went wrong and ensure it cannot happen again. Eton College offers its sincere apologies to those boys concerned and their families. We deeply regret the confusion and upset this must have caused."

When was an accidental email sent to potential students of Eton that warranted a personal apology to each family?