Although the Big Brother game does not officially start until the players move into the house on premiere night, pre-gaming sometimes takes place prior to the start of the season, which can ultimately affect how the overall game plays out. Pre-gaming isn't super common but has come up a few times in the history of the show. While it happens more when a season includes reoccurring players, pre-gaming can take place during any Big Brother season.

https://screenrant.com/big-brother-pregaming-how-it-affects-the-show/