Quick Definition: Meeting a girl again after the initial pickup, usually on a different day.
Day 2s are commonly referred to as “dates” in non-PUA parlance. The reason why PUAs often refer to dates as “day 2s” is to escape from the typical dating frame of taking out a girl to a dinner and a movie, with an awkward kiss at the end.
Rather, the typical day 2 involves a fun outing with opportunities for building rapport and kino escalation. Common day 2s include grabbing drinks at a coffee shop or a bar, or doing something active like playing billiards or mini-golf. It is important to consider logistics when planning a day 2, and choosing a location that is close to your house will make it easier to go for the close if things go well.
Building rapport and comfort during the initial interaction are important to get the girl to trust you enough to go out with you and not flake on the day 2. Also, making the day 2 request as casual as possible, without putting undue pressure on the target, will make it more likely for her to show up. Day 2 requests should be specific (as opposed to: “we should hang out some time”), and they should usually relate to a conversational thread from the interaction.