Party Train...?

I was at Omiya Station (Sataima, wah!) waiting to catch the Asama Shinkansen to Nagano a couple of Sundays back when I saw this sign.

Nothing unusual, right? Just a couple of signs telling you when and what the next train will be at the platform. But look closer at the sign on the right side.

What is a "party" train?

Wonder how the "party" was. Can someone explain, please? =P