1. In 1785, inventor and statesman Benjamin Franklin disclosed his design for a new musical instrument, the glass armonica, to the president of the Royal Society in London. The glass armonica is played by rubbing one's wet fingers on the rims of various-sized glass bowls that are arranged in a particular sequence.

2. In 1911, the Triangle Shirtwaist Factory Fire occurred in New York City, killing 146 garment workers, most of them young immigrant women. The fire led to significant changes in labor laws and workplace safety standards across the country.

3. In 1934, the Apollo Theater opened in Harlem. The venue quickly became a hub for African American culture and played a significant role in launching the careers of notable musicians, such as Ella Fitzgerald, Billie Holiday, and James Brown.

4. In 1965, Martin Luther King Jr. led the Selma to Montgomery march in Alabama, which aimed to secure voting rights for African Americans. March 21 was the first day of the march, which ultimately helped to usher in the Voting Rights Act of 1965.

5. In 2006, the first Twitter message was sent by co-founder Jack Dorsey, which read: "just setting up my twttr." Twitter has since become one of the most popular social media platforms globally, with over 300 million active users as of 2021.