Usually a planned action with: tomorrow, this month, this year, next month, next year etc
I am buying a car next month
Note, we can use “to be going to construction” for that too.
I am going to buy a car next month
So, what is the difference?
I am going to buy a car next month – I intend to buy a new car next month
I am buying a car next month – It is already decided and arranged, planned
Also on Present Continuous: