Why does Christmas sometimes skip falling on certain weekdays?

That can happen with any date and is due to a leap year causing a day to be skipped. The longest gap is 11 years. For example, Christmas Day was a Saturday in 2010. It will be a Friday in 2015, but because 2016 is a leap year, it will skip over Saturday and be on a Sunday in 2016. Saturday will come round again in 2021, which is the next time Christmas Day will be on a Saturday.