You can’t measure it easily. Velocity’s value comes from its inherent consistency. A fixed iteration length helps drive the reliable rhythm of a project. Without this rhythm, you are constantly revising, re-estimating, and reconciling, and the ability to predict out in the future is minimized due to inconsistent results.
If, on the other hand, almost everyone is going to be out a week for the holidays or a couple of days for company-wide meetings then, by all means, adapt iteration dates or velocity accordingly. Like most agile practices, these are guidelines, not strict rules.