I don't work Fridays/ I don't work on Fridays. Which one is correct?

Hi everyone,

Just a minute ago I heard on television:

  • I know you don’t work Fridays.

Did I hear this correctly and can you also say:

  • I don’t work on Fridays.?


If I remember properly, I came across it in a novela by a nowadays British writer. So, NES talk that way too, not using the preposition “on”. Could be informal English, I am not sure.