Curate, connect, and discover
Imagine the boys hanging around today and asking Nightmare where Cross is since they haven't seen him. Nightmare tells them Cross asked to be taken to a surface au today, that this was a holiday in his home au when they got to the surface and it seemed like he wanted to observe it in peace so Nightmare left him to it. None of the others ever saw the surface in their own worlds so they shrug it off, assuming it's probably some solemn thing he wants to attend out of respect as a royal guard.
Smash cut to Cross sitting in a park eating as many easter eggs as he can