What does Boxing Day have to do with boxing?
Happy as a clam refers to its being dug from its bed of sand only at low tide; at high tide it is quite safe from molestation.
1830+; orig. happy-as-a-clam-at-full-tide
Very happy; euphoric: She's happy as a clam with contractor Roe Messner/ on a posing dais in front of a full-length mirror, happy as a clam
[1636+; fr earlier locution happy as a clam at high tide, that is, when it cannot be dug]