Not sure what you mean by break between cycle and post cycle therapy (pct), as when you are finished with your cycle you should be working towards PCT and that includes continuing to eat healthy, training, and if you're running an A.I. such as Adex running it until 2 days before PCT.
Anywhoozle, the simple point of view is easy to understand and follow: When doing a cycle commit to all aspects of it and run it to completion. Cycle will include while on, and through PCT. Commit to your cycle, your diet, your rest, and your training. No drinking.
On the other hand: No one drink is not the end of the world, and will not destroy your cycle, nor will it destroy your PCT. Not many can just have the one drink though. Kicking it up a level, a controlled one night of drinking will not kill the cycle, but I'm guessing most will have more than just the one night. And typically will over do it in that one night, as well as the other nights, as if their level of control and discipline was high enough to limit to the one drink, they would likely not be drinking on cycle.
Wait until the end of your cycle if you don't have the discipline to limit yourself to one night / one drink. Although I wouldn't drink on cycle, unless as some mentioned it was a special occasion such as my friend getting married, not just some normal night that can wait until after my cycle.