Custom column week day

I have a very simple table with a list of dates in 'date' format.

I want to create a custom additional column, that takes the 'date' from the first column, and simply converts it into whatever the day of the week that date values falls on. For example, 'Monday', 'Tuesday' etc.

I have tried to do this using online tutorials, but to no avail. It always tells me that whatever function I use (for example, 'FORMAT') is not recognised.

To be clear, I'm trying to do this in Query Editor by selecting 'add custom column' and then punching in a formula into the box.

Thanks!