Target individuals contemplating change
How does change happen? It happens very slowly, maybe only starting with one action.
That initial action might have been a spurred by a trial coupon, a recommendation from a friend, or getting lost and discovering a sign. These actions that inspire change happen at an individual level.
One important thing to remember: advertising can influence behavior but a person has to want to change. Without the will to act on the action your advertisement suggested, there won't be any change. When people are most receptive to the nudge that gets them moving? A lot of factors come into play, like time of day, message content, interest, etc.
A good indicator is that they will be in the contemplation stage (according to the Transtheoretical Model of change). Pre-contemplation won't care about what you have to say since they are not engaged yet.
In the contemplation stage, maybe they've thought about trying your product, visited your site without subscribing t a talked to some friends. They are thinking about it, but haven't made a plan yet.
Reach out to those contemplating a change when they are ready and see what they respond to most. It's amazing how much wording alone can make a difference in receptiveness. Refine your message and then keep delivering.