Jesus told his disciples to wait, as he was going to send them a gift to help them make disciples of all nations before he ascended into heaven (beat death). They were very sad, but obeyed his instructions.
WOW... GOD REALLY CAN DO SO MUCH STUFF FOR US...
So the disciples waited, and waited, and waited. Even when there was a giant party going on outside! This was called the Pentecost.
SO THE DISCIPLES AREN'T DRUNK?!
MAYBE I SHOULD CONVERT TO CHRISTANITY...
Suddenly, there was this big wind coming in from outside. Then fire came down from heaven, and landed on top of people's heads! This was the holy spirit.
...AND THAT WAS THE END OF PENTACOST
Suddenly, the disciples found themselves speaking languages that they didn't even know (e.g. Arabic and Hebrew)! Many people thought this was wierd and that the disciples were drunk, but they knew that it was a special message. They used this to carry the message of god.
Even Peter, who had denied knowing Jesus, had made a moving speech about God (and to clear up the fact that the disciples weren't drunk). This speech was very successful and about 3,000 people became Christians.
...And that was the starting point of one of the most successful religions in the world- Christianity. For Christians, Pentecost is important to Christians as it was the starting point for their religion and everyone who believes has the holy spirit.