This says absolutely nothing other than the central limit theorem exists. Christmas is not seemingly any more or less deadly than average.

Between any given day, the amount of murders that happen is going to fluctuate a lot due to the relative rarity of murders. If you average lots of these days, or 365 of them in the case of a year, it converges toward an average.

You can think of each day as a "sample". Lots of days is a larger sample. This convergence is explained by the central limit theorem.

https://simple.wikipedia.org/wiki/Central_limit_theorem