It was a year after your adventures. You broke reset, load and save buttons for others sake. And now you need to be careful since you only live once.

What you hardly remember Sans invited you again into dinner at MTT hotel

Back from flashbacks, You're sitting in dinner table with him. As he pulls out acoustic guitar out of nowhere and plays "It's raining somewhere else". Sans invited you due to now vacation and you done all your homework

Then at middle length of ost he, then teleports himself onto piano seat and continues to play this ost.

He then tells about his promise with Toriel, blah blah

It was so boring that you almost fell asleep. Sans then finally finishes playing it as he sits back onto dinner chair as he speaks to you

Sans: didn't know that you got so bored quickly