They were doing a system test on Saturday, 26 April 1986 when a sudden unexpected power surge happened, They started a emergency shut down causing a larger spike in power output.
After the large spike in power it led to a reactor vessel rupture and a series of steam explosions that exposed the graphite moderator to the air causing it to ignite.
The city of Pripyat wasn't told about the disaster until 2 days after the 1st explosion. The world only found out when a radiation detector went off in Sweden.
For $250 dollars you can book a trip to the exclusion zone. While this is safe the director states that the area will be inhabitable for 20,000 more years.