The halving will occur at blockheight 840000. Since blocks come in every 10 minutes ON AVERAGE it is impossible pin-point it exactly. Even when block 839999 is mined it can't be determined when block 840000 is mined. Could be a minute, could be an hour. But in 10 minutes on average 😉
Tip: open up a tab, go to mempool.space and observe it during the day having a peek once every so often. You will see blocks in rapid or slow succession, but the average is 10 minutes 👍