What is the halving?

Each block pays the miner who found it a subsidy of new bitcoin, plus the fees in that block. The subsidy started at 50 BTC. Every 210,000 blocks it is cut in half. That event is the halving. It is enforced by the nodes checking the block, not by a company announcing it.

The schedule so far

Block 210,000 (28 November 2012) cut 50 to 25. Block 420,000 (9 July 2016) cut 25 to 12.5. Block 630,000 (11 May 2020) cut 12.5 to 6.25. Block 840,000 (20 April 2024) cut 6.25 to 3.125, which is the subsidy now.

The next cut is at block 1,050,000, down to 1.5625 BTC. The block height is the rule. The calendar date moves if blocks come faster or slower than ten minutes, so it is not a date you can put in a diary as a fact.

Why the cap is 21 million

Add the series. 210,000 blocks at 50 BTC, then the same number at 25, then 12.5, and so on. Ideally that is 210,000 × 50 × 2 = 21,000,000 BTC. In practice the subsidy is counted in whole sats and rounded down at each halving, so it reaches zero after 33 halvings and the total lands just under 21,000,000 (real cap: 20,999,999.9769 BTC). The last sats are issued around the year 2140.

After the subsidy

Miners are also paid the fees of the transactions they include. As the subsidy falls, fees become the larger part of that pay. The block is still limited, so fee rate still decides who gets space when the mempool is busy. The halving does not change the 21 million cap, the 600 second aim, or the 100,000,000 sats in a bitcoin.