This tense can be used with a time expression for a definite future arrangement:

e.g.  The boys start school on Monday. / e.g. I leave tonight.

instead of the more normal present continuous tense:

e.g. The boys are going to start school on Monday. / e.g. I’m leaving tonight.

The difference betwee them is:

a) The simple present is more impersonal than the continuous. ‘I’m leaving tonight’ would probably imply that I have decided to leave, but ‘I leave tonight’ could mean that this is part of a plan not necessarily made by me.

b) The simple present can also sound more formal than the continuous. A big store planning to open a new branch is more likely to say:

‘Our new branch opens next week’ rather than ‘ Our new branch is opening next week’

c) The simple present is sometimes used where the continuous would sound a bit clumsy, e.g. when speaking of a series of planned  future actions, particularly when they refer to a journey; we say:

‘We leave at six, arrive in Dublin at ten and take the plane on …….‘     instead of:

‘We are leaving at six, arriving in DUblin at ten and taking the plane on…..’

From:

A Practical English Grammar, A.J. Thomson, A.V. Martinet, 1994, Oxford University Press

This grammar is available at the school’s library. It is highly recommended to solve your grammar doubts!.