Q:

Why is the present simple tense used for an event that is in the future?

I would like to know the difference between "be going to," the present continuous, and the present simple. We use one of them when we talk about future, but I can't differentiate them. For example:

The holidays start next week.

Why can't we use: 

The holidays are going to start next week.
The holidays are starting next week.
The holidays will start next week.

A:

Using the present tense for future events indicates certainty, consistency, and familiarity.  In other words, use this to talk about events which will happen, which happen on a regular basis (or are predictable in some way), and about which you have some personal knowledge.

The holidays start next week.

I know this happens every year, last year I saw it happen in the same way, and I am sure it will happen the same way this year.  

The train leaves in five minutes

I know the train leaves at a specific time, I know that trains usually leave on time, and I have seen the schedule to confirm when the train will leave.  

The gala event starts tomorrow

I have personal knowledge that it is scheduled for tomorrow, I know that events like this usually start on schedule, and I feel comfortable asserting that it will definitely happen.
If any of these three does not exist, there is uncertainty, and the future tense will be more appropriate.  

We get paid on Friday

I know this happens with some consistency and regularity, as I've already received at least one paycheck on Friday.

We will get paid on Friday

Oops, now there is some doubt.  I hope this will happen, but I'm not sure it'll happen.
Here's another example: given no other context, consider these two sentences:

He will make the winning shot of the game in twenty seconds.
He makes the winning shot of the game in twenty seconds.

The first indicates a future condition, but the context is unclear.  Do I really know this will happen?  Probably not.  More likely I'm expressing an opinion, or a personal hope.
But the second sentence, that expresses an odd certainty.  The context is either that we are watching a recording of the game which I have seen before, or that I'm psychic, and I've already seen this happen with my mental powers.

A:

When we are sure about the event/things in the future, the present tense is okay. 

The holidays start next week - it's fixed that the holidays are coming next week. It's similar to the Valentine's Day is on February 14 and not will be on*. 

Some more examples - 

The train leaves in 5 minutes - the train is scheduled to depart at that time.
  
  The gala event starts tomorrow - it's fixed that it'll begin tomorrow.

