Does the Census Bureau make any adjustments to its monthly retail estimates to account for leap year?
Yes. The Census Bureau's seasonal adjustment software, X-12 ARIMA, handles leap year as part of the trading-day adjustment for the monthly retail sales estimates. When using a multiplicative model and the td (trading day) option, the February estimates are rescaled before applying the log transformation. This rescaling multiplies a given February's estimate by the ratio of the average length of February (28.25 days) to the length of the given February (28 or 29 days). No trading day adjustments are applied to end-of-month retail inventory estimates; therefore, these estimates are not adjusted for leap year.
Page 39 of the X-12 ARIMA documentation provides a similar description of the leap year adjustment.
For more information, visit the Monthly and Annual Retail Trade website.