The Holy See formally established the congregation in 1820. This, too, was an unusual event, since the Christian Brothers were the first Irish congregation of men approved by a charter from Rome.

Answer this question: when did the pope give final approval for the congregation of the christian brothers? Let's think fast. Stream of consciousness:
To answer the above question, the relevant sentence is: The Holy See formally established the congregation in 1820. So the final answer is 1820.