You only train once on fridays, but you can change the training until this moment all the times you want.
While this is completely true, bear in mind that every time during the week (the week defined when you click the "Weekly Stats" link on the long menu), the minutes your players play are going to that eventual training on Friday (which takes place early in the morning btw).
From reading in other parts of the forums, you need to give whatever players you want to train that week at least 48 minutes to receive
fulltraining for that week; the general consensus is that you do receive partial training for less, but that it is not a linear decrease, that is 46 minutes does not translate to 98% training.
Also, while not directly related to training, there seems to be somewhat of a consensus that it is a good idea to try to play your players around 70 mins a week (im saying that as a middle from the two extremes i have seen) in order to maintain and increase your game shape.