You have three options. (a) Tell him confidently that you're sorry but you can't help him until Monday. You know he'll respect you for it. (b) Blurt out "no", slam the phone down then spend all weekend worrying that you've kissed your bonus and chances of promotion goodbye. (c) Quietly agree to give up your weekend - hating your boss, your job and yourself as a result.
Chances are most of us usually tick the (c) option. But one of the keys to achieving a healthy work-life balance is learning how to say "no" without suffering any ill effects. A warning, though - a Canadian academic, Professor Robert Hare, has done research that shows there are more "sub-criminal" psychopaths (self-serving narcissists with no ability to empathise with other people) than we realise and many of them get into management positions. If you suspect your boss is a psychopath just get the hell out of there.
If your boss is reasonably normal, the key to setting boundaries and saying no lies in knowing yourself and building your personal credibility. Self-awareness is critical. Most of us want to please other people but sometimes can't stop saying yes -even if we'll regret it later - so it helps to take a step back and analyse why we feel the need to accede to every demand that's made of us.
'How To Say No: 200 Ways To Say It And Stop People-Pleasing Forever' by Susan Newman explores different scenarios at home and work where you may get hit with an unreasonable request and unpicks what's really going on.
But as well as knowing yourself, it's just as critical to build your "social capital" at work - to be seen as the kind of high-integrity person who can say no with credibility.
Most people hate surprises in the workplace. So suddenly saying no to a boss is likely to shock them. The answer is to sweeten the pill by saying "I know X won't mind getting that project a day after the deadline, and I'll be able to give it my full attention when I get in on Monday". It is, however, easier to get your positioning right when you start a new job than playing catch up in an established situation. Oh, and take it from me - option (b) is never a good idea.
Steve Crabb is editor of People Management