STO delivery schedule

Hi Experts,

Please giude.

Here i am creating STO today so as per plant delivery time (Days), system is determining the Delivery date but after ATP check run system is showing some other date

Ex. PO creation date = 15.01.2014

Planned del time = 2 Days

Delivery date = 19.01.2014 (Considering friday as holiday in Factory calender).

But for 19.01.2014 system is not committed the Quantity

and determine another date 21.01.2014 and comits the Quantity , i know this is the dat determine by ATP check and comits the Qty but how this date

will arrived i.e 21.01.2014?