Update: As of F1 2020's release this Achievement may now be unobtainable, as Weekly Events become less frequent.
Weekly Grand Prix is one of the events you can complete in Multiplayer.
These typically happen in the days leading up to a Grand Prix weekend; you can find the 2019 season calendar here:https://www.formula1.com/en/racing/2019.html
When a Weekly Grand Prix is active, you will have a number of days to complete Practice and Qualifying sessions, before you can take part in the Race on the weekends.
Races kick off at 1 hour intervals, so make sure you're ready to matchmake shortly before the top of the hour.
Assuming you have completed Practice, Qualifying and the Race in asingle Weekly Grand Prix, the Achievement should unlock.