spätestens in English:

I'll call you on Friday at the latest.

During a day, we should eat five meals, first meal at the latest two hours after waking up and the last meal three hours before we go to bed.

I'll send it to you at ten at the latest.

Let me know on Wednesday at the latest.

Please pay this bill in ten days at the latest. I'll be home by midnight at the latest.

We have to be in New York by Monday at the latest.

Call me on Tuesday at the latest.

I do this by the end of next week at the latest