New bitcoins are issued by the Bitcoin network every 10 minutes. For the first four years of Bitcoin’s existence, the amount of new bitcoins issued every 10 minutes was 50. Every four years, this number is cut in half. The day the amount halves is called a “halving”.
In 2012, the amount of new bitcoins issued every 10 minutes dropped from 50 bitcoins to 25. In 2016, it dropped from 25 to 12.5. Now, in the 2020 halving, it will drop from 12.5 to 6.25.
The Bitcoin halving is scheduled in block height, not date.
The halving happens every 210,000 blocks. The 2020 halving will happen on block 630,000. The 2024 halving will happen at block 840,000.