Future Simple and arrangements

Remember that you should never use will to say what somebody has arranged or decided to do in the future:

• Mike is moving to New Jersey next month. (not "Mike will move.")
• I'm going to visit my friend tonight. (not "I will visit my friend tonight.")
• Paul isn't free on Thursday. He's working in his aunt's shop.

As you can see, the Present Continuous and going to are used to express future arrangements.