It means to declare the end of the working day (not a literal day, but the length of time needed to do the job).

Thanks Ksenia

This informal idiomatic expression means to stop doing something, especially working or studying. E.g. I'm getting a bit tired now - let's call it a day. // The boss was mad because Tom called it a day at noon and went home. // After playing together for 20 years the band have finally decided to call it a day.