The block subsidy began at 50 bitcoin every ten minutes.

Every 210,000 blocks, the subsidy is cut in half until we reach just under 21 million bitcoin around 2139.

There are 144 ten minute blocks in a day.

52,560 per year.

210,248 ten minute blocks every 4 years, so we have a halving every 4 years and a couple days. The math is easier than it looks.