As I’ve had occasion to say in answering other questions, it depends on what has gone before and, to some extent, what comes afterwards. English expresses the future not only by will be or going to followed by the plain form of the verb, but also by the present tense. These forms are used in different contexts.
Will + the plain form of the verb is used for simple prediction about something we know will happen or which we expect to happen. In response to the question ‘Hey, does anyone know what day of the week the 1st of January falls on?’ the answer might be ‘I can’t tell you which day, but one thing I know for sure: the next century will begin on the 1st of January 2001.’
Going to is used when an event is pre-planned or inevitable. If someone had said on 31 December 2000 ‘I’m worried about the future. I wish I could stop the clock', the response could well be ‘The next century is going to begin on the 1st of January 2001, whether you like it or not.’
The present tense is normally used to refer to scheduled future events and future certainties, particularly in subordinate clauses. It could have been used if someone had asked ‘What are you doing on New Year’s Day?’ The answer might be ‘Something special, because the next century begins on the 1st of January 2001.’
There’s a fourth, but less likely, possibility, and that is the present progressive construction. In December 2000, someone might have said in a speech 'The next century is beginning on the 1st of January 2001. Let us therefore take this opportunity to prepare ourselves for a new age.'