PO warning message

Hi.

When I first time change the PO delivery date and press enter without save the PO, system will display warning message base on SAP calendar setting. Let say 1st, 2nd and 3rd September 2008 is already set as non working day and 4th September 2008 is a working day. So for the first time if I choose 1st September and press enter system will display below warning message:

Delivery date: next workday is 04.09.2008

But when I change the date to 2nd September 2008 and press enter, system will not display above error. My question is this scenario base on SAP Standard?

Thank you.