Treasury Regulation § 20.2031-2 (b) 1 addresses inversely weighted means. Means are inversely weighted with respect to the number of trading days between the valuation date and pricing dates.
Since markets were closed, for example, from September 11, 2001 through September 14, 2001, the weighting should not apply and means are derived from a straight average of the 10th and the 17th. The same applies for other market closures.