So I found this videosupposedly you go on a date with Sans but this is different than the Papyrus date and besides that, I never saw this in the game maybe, I have not gotten to this part. But I did go to Grillbys with Sans and I thought they were just hanging out as friends. So is the Sans date real or a hoax?
So it looks like you can get dinner with Sans if you go to Grillby's and talk to him before heading to the core. In most of the videos I've found in the descriptions it says "this is my favorite scene in the game" or something along those lines proving that it's in the game. Weather it's a date or not it up to you to decide really because they did go out to dinner but no one said anything about a date.
It isn't a date, as many have wondered. It's a progression in the story and shows character advancement for Sans & Toriel, showing a little more backstory. Sans doesn't like you, and personally, I believe even if you go through a pacifist route, Sans still won't like you. Because he knows your gonna leave. Even if it's your first time playing, he knows since hes gone through timelines before. Because eventually, curiosity will strike you and bam, genocide. However, this is a real scene in Undertale, which I believe is part of the main route but I've only played genocide and pacifist so I am unsure! Have fun. :)