Independence Day (15 September) and 15 August, the day of the national patron saint, María, are the most important national holidays in Guatemala. On 15 September 1821, the Captaincy General of Guatemala, formed by Chiapas, Guatemala, El Salvador, Nicaragua, Costa Rica, and Honduras, officially proclaimed its independence from Spain.

The Captaincy-general was dissolved two years later. This region was formally a part of the Viceroyalty of New Spain throughout the colonial period, but as a practical matter had been administered separately. It was not until 1825 that Guatemala created its own flag.

Source: https://en.wikipedia.org/wiki/Guatemala#Independence_and_the_19th_century_.281821.E2.80.931847.29