What was the name of the English law establishing an annual 5 November thanksgiving after the Gunpowder Plot failed?

The English law establishing an annual 5 November thanksgiving after the Gunpowder Plot failed was the Observance of 5th November Act 1605.

The Act required an annual public thanksgiving service in England on 5 November, commemorating the failure of the 1605 plot to destroy the House of Lords and kill King James I. It was passed by the Parliament that assembled after the discovery of the conspiracy.

Although commonly called the Observance of 5th November Act 1605, the legislation was enacted in 1606. Its formal purpose was religious and political: to thank God for saving the king, Parliament and the kingdom. The Act helped turn the anniversary into an official national observance, alongside bonfires, bells, sermons and public celebrations.

The legislation remained in force for centuries. Parliament repealed it in 1859, but 5 November celebrations continued as a popular tradition. The date is now more often called Bonfire Night, Guy Fawkes Night or Fireworks Night, depending on local usage.