when should I tell my boss I'm going to quit working for him? because I'm going to start my own business.

I have worked for him for almost 2 1/2 years, and I'm going to start my own business in january, so I will be his competition by then,t he is going on vacations on december, he totally relies on me and a coworker for his business, I forgot to mention that, that co-worker is going to work with me, our boss doesn't pay us enough, he is a good boss but really underpay us, he is coming back from vacations on the firs days of january, I would like an advise on when t tell him before he goes on vacations or when he is on vacations. what should I do?