Requirements date for VSF -- why not on first of month?

Please help me understand this peculiar system behavior.

MD04 routinely displays the plant/materialu2019s planned independent requirement as a u201CVSFu201D having a requirements date of the first work-day of the period.

What I cannot explain is why the October 2012 VSF has a requirements date of September 21, 2012. Why would the date not be October 1, 2012? The factory calendar assigned to the plant has October 1, 2012 as a work-day.

Same situation for December 2012. The VSF for December has a requirements date of November 21, 2012.

For the rest of 2012, and for the first three months of 2013, the VSF elements are dated on the first work-day of the month. What would cause the requirements to fall a week and a half early for October and December 2012?

By the way, I am not very fluent in MRP, so please do not assume that I have ruled out something obvious.

Regards,

zKen