With default settings, the clock on macOS Catalina shows the current day of the week and the current time. You can click the clock to reveal the full date.
In the Big Sur update, this dropdown seems to have vanished, instead opening notification center on interaction.
I don't want to see the full date at all times. It takes up space in the dock, and most of the time the full date is not really relevant information. I would prefer to be able to use the old behavior if possible.