The two-hundredth anniversary of something is its bicentennial. In 1976, the United States celebrated its bicentennial, two hundred years after adopting the Declaration of Independence.
The word bicentennial adds the prefix bi-, or "two," to centennial, which is rooted in the Latin centum, or "one hundred." The U.S. is a relatively young country, so we've only celebrated a few bicentennials, including the two-hundredth birthdays of George Washington and Abraham Lincoln. You can use this word as an adjective too: "Next year is the bicentennial anniversary of our ancestors' arrival in North America."