What does the word convention mean as it is used in this sentence? it was the convention in our family to tell jokes at Thanksgiving, but my new brother in law had always had serious Thanksgiving meals with his family.

In this sentence convention would mean a way in which something is usually done

Behavior that is normal and common. Considered conventional.

Explanation:

Gawian told Arthur not to attack Mordred, he obeyed Gawain but when he sent his army for a treaty to Mordred, Mordred did not believe the treaty. Arthur and Mordred doubted each other which led Arthur to attach despite the warning.