Arthur is a son (no daughters) born into a pure-blood wizarding family.
The month and date are stated in canon. The year is an approximation, hence the "circa" in front of the date. It is based on Arthur and Molly eloping at the start of Voldemort's first rise to power around 1970 and before the birth of Bill in 1971. Assuming that they were just a year or two out of Hogwarts when they eloped, that would put their Hogwarts years as approximately 1961 to 1968, and therefore their birth year as approximately 1950.