The Portes Ouvertes in the Médoc are usually the first major event of the year in terms of "open doors" at wineries.
In 2023, they were held on the weekend of 1 and 2 April.
And here's a link to the event's official website.
Video trailer for Portes Ouvertes des Chateaux du Médoc 2023
And here is a short video-trailer for the Portes Ouvertes du Médoc 2022 ...
... and one more for 2021 (moved from the usual April to June due to covid-19)
If you want to get to know the Médoc appelations in detail, I recommend this video: