Since when was there a party on late night?

"Bringing the Party back to late night?"

It implies that at some time or another there once was a "Party" on late night.

What "Party" could he possibly be referring to?

Certainly none of these late night stiffs qualify as a "Party"!

Not Jay.
Not Dave.
Not Conan.
Not Jimmy.
Not the other Jimmy.
Not Johnny.
Not Earvin.
Not Pat.
Not Joan.
Not Bob.
Not Carson.
Not Joey.
Not Garry.
Not Jack.
Not Craig.
Not the other Craig.
Not Dick.
Not Greg.
Not Bill.
Not Jon.
Not Steve.
Arsenio, maybe.
And finally the original. Steve! He was the first to do a late night show. But it was so long ago, who remembers?

And then there was -
Hugh. That actually WAS a Party, but it doesn't really qualify. It was only weekly and was short lived at that.