The concert will start tomorrow at 6:00 pm.
The concert starts tomorrow at 6:00 pm.
I feel certain there is an answer to this question already, but I can't find it. So, I will answer it briefly.
The concert starts at 6PM tomorrow.
The concert is starting at 6PM tomorrow.
The concert will start at 6PM tomorrow.
All of these convey the same timing and are all grammatically/stylistically correct.
If you are forced to nit-pick, typically present tense is used when you are mentally already in the future. In other words, if you very excited for or very weary of a future event, you are more likely to use present tense. A simple future might imply a sense of indifference: you are stating it without getting involved in the event emotionally. Once again, I cannot state enough that this is nit-picking. There's no such "rule".
So, a concert promo poster might use present tense to convey a sense of excitement.
Concert starts at 6PM sharp! Bring your friends!
On the other hand, a safety bulletin might use future tense because it is meant to be conveyed as a matter of fact, and without emotion:
The concert will start at 6PM. We will make an announcement at 5:50PM informing the attendants of all the fire exits.
(1) The concert will start tomorrow at 6:00 pm.
(2) The concert starts tomorrow at 6:00 pm.
If you know for a fact that the concert starts at the scheduled time, always use (2). Under normal circumstances, this will be the case. So it's the more natural.
If, for some reason, however, you're not certain about the concert schedule, (1) will sound better than (2).