Sometimes the position of nicht can change if you want to emphasise a point. Look how the different position changes the meaning slightly.

• Wir fahren am Sonntag nicht zu Opa. – We are not going to grandad's on Sunday.

This stresses that we are not going to grandad's, although we may be going somewhere else, eg uncle's house.

• Wir fahren nicht am Sonntag zu Opa. – We are not going to grandad's on Sunday.

This stresses that we are not going on Sunday, although we may go another day.

• Sie sind nicht mit dem Bus in die Stadt gefahren. – They didn't go to town by bus.

This implies that they did not travel by bus, but travelled by another means of transport. It does not negate the fact that they went to town.

• Sie sind mit dem Bus nicht in die Stadt gefahren. – They didn't go to town by bus.

This suggests they went by bus, but not to town. They went somewhere else, eg the countryside.

• Ich will nicht mit dir auf die Party gehen. – I don't want to go to the party with you.

This suggests that I don't want to go to the party with you, but I'll go with someone else.