The best time to stop smoking is when it feels right for you. Don't let anyone tell you it can't be done, including yourself. The problem with setting a date for the future is that it stops you from dealing with it now. It also doesn't allow for the possibility that maybe you won't want to stop that day. Forget significant dates like birthdays, New Year's Eve, Christmas etc and stop when it's right for you. That's not to say that those dates aren't OK to stop on, they may be right for you. Ideally make that today, you're obviously already thinking about it.
Every year, thousands of people stop smoking on a certain, pre-planned day and only manage to stay stopped for a few days. But don't worry if this happens to you. Don't beat yourself up or see it as a failure. You may have to try two or three times before you stop smoking permanently. View it as a platform for future success.
For further ideas and advice and ways to stop smoking, visit the substantial resources section of this site - Stop Smoking Resources