Due to Unfortuante Timing, the Cat is Out of the Bag

My boss knows.
This morning as I was making my daily bowl of oatmeal in the breakroom, my boss (Mark) pops in and says "Would you like to hear the bad news or the worse news?". My first thought is that everyone at the Bank is going to be laid off. Then I think I'm going to be laid off. I just stare at Mark and answer "Let's go with the bad news first".... He proceeds to tell me that our other bank administrative assistant (I am senior adminstrative assistant) gave her notice (bad news) and that her last day is Thursday (worse news). My heart drops. This totally ruins my plan of when to tell Mark that I'm leaving. What is the Bank going to do with no admins??
I then proceed to tell him that I was going to give him my notice... He asks if I'm serious... Yes, Mark..... My last day is June 23. I can literally see stress lines start to form on his face. I feel TERRIBLE. Mark is a great boss and an awesome employer. I go into his office and we start talking about my surprise announcement. I wasn't planning on telling him until May 27 or June 10.
Throughout the day, Mark is trying to make suggestions to me to get me to stay a bit longer, each comment is about 75% serious... like chaining me to the desk and building a port-a-potty next to the desk. Don't worry, he'd bring me food. Or hey, don't I want to stay until I actually have a job in California? No Mark.... I'm sorry.
Mark knows that this nothing personal against him or the Bank. I told him that on the bright side that I am here for another month. So he has gotten approval from an EVP (Sallie Mae is on a hiring freeze) to fill my and the other admins position. I sure hope that I am here for the interview process and hopefully can train at least one person.
I must say I have enjoyed my time at Sallie Mae Bank. It was a challenge majority of the time because I felt I was worth more. But I highly enjoy the people I work with and the atmosphere that is there. Mark is someone who treats all his employees with respect and does not act as if he is superior than anyone. Even the plant lady. I hope that wherever I end up in California, I would be so lucky to have an employer like Sallie Mae Bank.