Close of Play
Or Six or whatever time is considered the end of the working day in your office. Unless your place of work clocks you and your colleagues out at the same time every day, then it is hard to know the exact time intended.
This one has two theories. Some say it comes from financial markets, meaning the end of the trading day. The time implication makes sense here: when the markets close is a natural way to view the end of the working day.
But the expression does use the word “play”, implying an athletic provenance. Most likely it was absorbed by the financial markets from a sports origin, and then entered standard business parlance.
This part of the post should be easy to anticipate: be specific. What do you need, by when and why. If you want something before you leave your work desk that day, then say “I will leave my desk at 6PM today, please have this to me by 5.59”.
Having said that, if that is the case, then we all know you will not look at or use the requested task, document, report, etc. until the next day, so you may just say “tomorrow morning”.