Who decided that Christmas day should be 25/12?

It just so happens that 21/12 is the longest night of the year for the northern hemisphere. That's why Christmas should be 21/12.<br><br>Why is that?<br>Would the 25th be a better day for Christmas or something?<br>What's the reason for it being on the 25th?<br>That's when the winter solstice is?