I’ll take a stab at it… not quite sure how to articulate the concept though.
I think the idea of having a plan for the evening is changing. Back in the day, you tended to agree to meet somewhere well ahead of time, and that initial meeting place was more or less fixed in stone, as getting the word out about changes in venue or time was kind of tough, and someone always got left out. From there, you usually made up your mind as a group, in person.
Nowadays, it seems much more flash-mob like, with people texting in the early evening on a Friday about what’s going on that night, and it just sort of coagulating into a sort of ad-hoc plan.
I’ve seen it change from college (no cell phones), when unless you were just winging it with people from your apartments or dorm, you generally made a plan and stuck to it, at least for the first place you went for the evening. As a young adult, cell phones were common, and so was email at work (not texting though), and people would generally bash a plan together on Friday afternoon at about 3 o’clock, and usually stick to it, and notify anyone via cell if there were changes.
Now, as a middle-aged old fart, if I wanted to go out, I’d probably just text my friends at about 5-6 on a Saturday asking what they were doing, and if they didn’t have a plan, we’d probably agree to meet somewhere. Any changes would just be texted on the fly.