What was the official name of the 1605 law establishing an annual 5 November thanksgiving in England?

The law establishing an annual 5 November thanksgiving in England was the Observance of 5th November Act 1605.

Parliament passed the Act after the failure of the Gunpowder Plot, when conspirators planned to destroy the Houses of Parliament during the state opening. Its formal title referred to the “fifth day of November” and the deliverance of King James I, the royal family, and Parliament.

The Act required public thanksgiving services and encouraged people to remember the day each year. It helped turn an immediate political and religious response into a recurring national observance. The law was later repealed in 1859, but 5 November celebrations continued through bonfires, fireworks, and public gatherings.

The Act is sometimes casually called the Thanksgiving Act, but that is not its formal title. It also did not create every modern Bonfire Night custom; many traditions developed gradually in different towns and regions.