Hello Mahendra
On the process order (transaction COR2) you can change the scheduling type to backward or forward scheduling for an specific order. For example, if you want to have your order starting on 15.12, you should remove the basic finish date, set the basic start date to 15.12 and set the forward scheduling.
Please note that, on the customizing transaction OPUZ, start in the past must be allowed.
The same procedure is valdi for backward scheduling, but you have to set the basic finish date to 15.12 instead of the start.
BR
Caetano