I knew I wanted to quit and tried on my birthday in June. That worked for a couple days till I invited my ex over for dinner. She wouldn't usually do this, but she brought drinks. I sipped one out of politeness? but she left without taking the better part of a 12 pack. That got me going on a pretty long bender.
The next significant date I could pick was the 4th of July, Independence Day. I don't think I'd need a special date to quit, but it helps. There's more pressure that way, but also more guilt and feeling of wastefulness if I were to slip up again. It's like a New Years resolution, only not on New Years.