“Guys, let's call it a day.”

You're working on a project with a few other people at work. You've all been working hard all afternoon, and now it's getting late. You think you should stop working on it for today, so you suggest:

Guys, let's call it a day.

guys

In English, there's not a really good word for addressing a group of people. In different English-speaking areas, people use different expressions when talking to a group. The expression that's most common for American English is "guys". For example, a coach can say to a team:

Great job today, guys. Keep up the good work.

"Guys" technically means a group of men, but it's usually OK to use it for mixed groups of men and women, or even for groups of all women.

let's (do something)

Make a suggestion using "let's ___" when you're pretty sure that the people you're speaking to will accept your suggestion. If you're not as sure, you can say "why don't we ___":

Guys, why don't we call it a day.

call it a day

This phrase, "Call it a day", means to stop what you're doing for that day. You use this when you're not 100% finished with everything you could do, but you're tired or ready to go home. Use this when you're working on projects with people, practicing for something together, or just to let people know when you're leaving work:

Jean, I'm going to call it a day. I'll see you tomorrow.