The first official meeting of the Royal Astronomical Society was held on February 24th, 1820. It was moved from its original date of February 8th to respect the death of King George III.

Sources:

https://www.rmg.co.uk/stories/topics/astronomy-19th-century

https://ras.ac.uk/about-the-ras/a-brief-history

https://ras.ac.uk/timeline