Most of what gets said on Christmas Day is logistics. Who is driving, when the turkey goes in, whether anyone has seen the batteries. The card on the tree says "Love, Mom" and that is about as far as it goes. The real things, the ones you think about when you are wrapping their present alone at midnight, rarely make it out.
A letter you write ahead gets around that. You pick the moment it opens: Christmas Eve after the kids are in bed, first thing on Christmas morning, or after dinner when the house finally goes quiet. You write it on a calm evening when you can think. It stays sealed until then.
The nicest way to hand it over is to make it part of the day. Write the link on a small card and tuck it in their stocking, or slip it into the card on top of their present. For a kid, it can be a letter from you they get to open themselves. For a parent or grandparent, it can be the thing you never say out loud at the table. For a best friend, it can arrive in the middle of their own family chaos and remind them they have people outside it.
If one morning is not enough, there is also an advent calendar version: a dated jar that unlocks one short note a day from December 1 to Christmas Eve, so the whole month leads up to the letter.