Context Jon, context. Paul is addressing those "foolish Galatians" who, subsequent to their receiving and believing a gospel of grace, were setting about works (amongst which Religious Observation) - as if that mattered for their salvation.
If one utilises Christmas (or Christmas utilises one) for the purposes of rememberance of what is (once true) the most monumental occurance in human history - bar none - then I fail to see the harm in it.