How does an election for Congress get certified, and when is the winner official?

The canvass is the official tally of every ballot, combining the totals from every precinct and voting method into one complete count. Certification follows as a separate step. Campaign Legal Center describes it as a mandatory and purely ministerial process, carried out at both local and state levels, that confirms the election process has concluded.

Each state issues a certificate of election and sends it to the Clerk of the U.S. House, who uses it to compose the roll of Representatives-elect.

Under the Twentieth Amendment, Representatives' terms begin at noon on January 3, and Congress assembles at noon that day unless it appoints a different day by law. According to House Practice, until a Member-elect takes the oath of office, that person does not enjoy all the rights and prerogatives of a Member and cannot vote or introduce bills.