It will vary from calf to calf. Some may go early , and some may go right on time or later. Most of the time mine usually fit in the window from the week before or after they are scheduled to calve, but there are always exceptions.
I've found that they'll typically be a little later with subsequent births.. a day or two.. bull calves often are gestated a day or two longer than their heifer counterparts too.. won't see anything too drastic though.