I think that it is not necessarily a fixed month. When I got my 330, I ordered it in January of 2003, and the dealer said it would be a 2003. When I got the confirmation and build date, they said that the factory was changing over to the 2004 model at the beginning of march, and I would have a 2004, and I do.
I think the date may vary by model (coupe, sedan, etc.) and also factory.