Where is it? Does it exist?

There is no 'Night to Remember' quest. It is a song and was the title of the launch cinematic.

*sigh* would have love it though if it was a sidequest or monster contract

There are those that expect this to be a quest in the Blood and Wine expansion, seeing as it may fit into the main narrative there. I guess we'll just have to wait and see if this turns out to be true...