Bitcoin which forked in 2017 into two chains, Bitcoin (BTC) and Bitcoin Cash (BCH), both are set to halve every four years. This major milestone for each chain reduces the block rewards in half.
Keeping track of both halving events is easy by checking these two pages, which both track the halving events separately for each chain.
Bitcoin Cash (BCH) is set to halve soon. You can keep track of the Bitcoin Cash halving using this countdown clock.
Bitcoin (BTC) is set to halve soon as well but is about month behind Bitcoin Cash. You can keep track of the Bitcoin halving using this countdown clock.