The first Bitcoin block mined after the genesis block was Block 1.
Bitcoin’s genesis block is numbered 0, so the next block in the chain is Block 1. It was mined by Satoshi Nakamoto on January 9, 2009, shortly after the network launched. This block followed the genesis block’s creation on January 3, 2009.
Block 1 is notable because it contains the first block reward that could, in principle, be spent under Bitcoin’s normal transaction rules. The 50-bitcoin reward from the genesis block is treated specially by Bitcoin software and is not spendable in the same ordinary way.
Block numbering is zero-based in Bitcoin, which causes a common mix-up: the first block is Block 0, not Block 1. Bitcoin’s chain is therefore a sequence beginning with height 0, followed by height 1 and later blocks.