Answer:
because in a moment is more important and better than the whole day, like if you get heartbroken, your just gunna carry that the whole day which isn't good for your health. that is why.
The pun is, "Santa arrived at the party just to the NICK of time." He's often referred to as "Saint Nick." The meaning of the pun, is, he made it to the party quick, or he made it just in time.
A i think hope this helps you