For me it was first thing. My 2 DDs usually require rousing from their beds in time to breakfast and do gifts before going out to a relatives for dinner or starting to prepare our own. This year I heard DD1 (10) (who all week has needed to be woken at 10 each day to save her languishing in her bed all morning) creep through to her sister's room to tell her to wake up and that it was Christmas day. DD2 (7) was heard to tell her sister to climb in bed with her and have a cuddle as it was too early to get up yet. They lay in bed whispering about the sunrise and the fact that it was Christmas until I called them through to bring their stockings into my room to open them.
A close second has to be how easily they have gone to bed tonight. A late excited night last night followed by a full and busy day today and they are worn out. Fingers crossed for a lie in in the morning. 