That's the cunning nature of addiction, to make us forget the reasons we wanted to quit, forget the turmoil our drinking caused, forget how sick we were during the last hangover. Anything to get us to resume our ways. This is the addictive voice, the one that overrides our rational mind.
What worked for me was to write out exactly what my intent was, to quit drinking FOREVER. It was important for me to put forever because the first time I quit...forever...I forgot, did I meant forever? or until I was feeling better?
As for when, there is no time like the present. Setting a date is just you putting off what you don't want to do, which is quit.