Halving will happen when the miners reach the block #630,000.
After the 2020 Bitcoin halving, the mining reward will be reduced from 12.5 to 6.25 BTC.
The first Bitcoin halving occurred in November 2012 and the second in July 2016.
Bitcoin halving is the event where the reward for mining new blocks is halved - this means miners receive 50% fewer bitcoins for verifying transactions.
Bitcoin halving occurs every 210,000 mined blocks.
As the Bitcoin supply is limited (21 millions), the last halving will be probably around the year 2140.
The third Bitcoin halving occurred yesterday, May 11th 2020.