Retailers say it can be difficult to measure their monthly sales results accurately on a year-over-year basis because of calendar shifts — sometimes a holiday falls in one month, boosting results, while the next year the holiday shifts into a different month, hurting results.

The most drastic case of this is usually seen in March and April, when the timing of the Easter holiday can help March sales and hurt April, or vice versa.

As monthly sales results came in today, numerous retailers, including Target, Saks, and Pacific Sunwear, said the timing of Thanksgiving (Nov 27 this year vs. Nov 22 last year) hurt their November results. The later Thanksgiving meant fewer busy post-Thanksgiving holiday shopping days in this year’s November period, they said, making sales figures look especially weak.

For instance, Target’s same-store sales fell more than 10 percent. It had warned that the calendar shift eliminated 7 holiday shopping days from November and pushed them into December, which would drag down November’s results. Saks and Pacific Sunwear also reported lower November same-store sales.

But the Thanksgiving shift was apparently not a big deal for industry leader Wal-Mart. While most retailers reported a drop in November same-store sales, Wal-Mart posted a stronger-than-expected 3.4 percent rise.

And in contrast to many others, Wal-Mart’s November monthly reporting period ended on Nov. 28 — Black Friday. That meant the discount behemoth had just one post-Thanksgiving day in November compared with most retailers whose November reporting periods ended on Nov 29 — giving them two supposedly busy post-Thanksgiving shopping days in the month.

Looks like low prices on food and toiletries are one way to trump a shifting calendar.

(Photo: Reuters/Nicole Maestri taken in a Wal-Mart store in Secaucus, NJ the week of Thanksgiving)