To put it simply - it's just too big of an event for the Church to squeeze its celebration into one day! The same is true of Easter. We need an octave, eight calendar days, to even scratch the surface when it comes to appreciating these mysteries.
I thought about this last night in relation to the first Christmas. I re-read Luke's account of how the shepherds were sent by an angel to find Jesus, "wrapped in swaddling clothes and lying in a manger" (Lk.2:12). It dawned on me that I had never stopped to truly put myself in the shepherds' place and imagine how they would have reacted. In my head I had always seen them kneeling in silent awe before the manger ... and then going on their way. But that's not what human beings do; it's definitely not what I would have done!
Once the shepherds found Jesus there with Mary and Joseph, wouldn't they have had a million questions for Joseph? Wouldn't the story of how they had seen the angel and then a whole angelic army have came spilling out? And for the long-awaited Savior to actually be right there in front of them! I bet their eyes just kept coming back to Him.
Scripture tells us that the shepherds finally left (Lk.20), but I have to believe that they returned over the next few days. How could they not? If angels had appeared to you and told you where the Savior was, could you keep from going back? Of course not. We wouldn't be able to think of anything else and neither could they. They wanted more! They must have looked for any opportunity to pay Jesus, Mary, and Joseph another visit. "Boy, there sure was some nice grazing land over on the other side of the city. And while we're out that way ..." (Anyone else feel the sudden urge to visit Jesus in the Tabernacle or Adoration Chapel?)
Christmas is a mystery and celebration that simply cannot fit into one day. Happy third day of Christmas!