Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for same is still unknown. The rate of gain of concrete compressive strength in higher during the first 28 days of casting and then it slows down. In 28 days, concrete gains almost 99% of its strength. So, we test the concrete compressive strength on 28 days.

For more details: https://theconstructor.org/concrete/why-we-test-concrete-strength-after-28-days/6060/