My comment won't help you, maybe... But if my boss asks me to do this, I just say that my family is more important. And if he does not agree, I will find a new job, and he has to look for a new employee. The thing is, I can do that. I am a software engineer with certain skills that are very hard to find. If I quit my job, my boss is in more trouble finding a new employee, than I am finding a new job. Not to be arrogant or something, I am just lucky to have a brilliant mind for exact science. The thing is though that you have to set priorities. I have a lot of priorities that come before my job. Family, health, sports, just interests in other things, after that, my job. Bosses should really stop thinking that they are important!

I would just say something like "Sorry, but I'm busy this weekend" or "Sorry, but I've already made plans" and wouldn't go into any more detail. But I think it depends a bit on culture, what's expected of you in your job and how eager you are to please your boss(es).

Vivian, we are in China.

So do it in Chinese way.

Refuse and being polite.

I've done something like this once. Guess what?

my supervisor was not angry at all.

On the contrary, since then, every time she wanted me to overwork, she asked me in a  very careful way.

This is the thing, you win the repect and your free time by yourself.

Just say, sorry, I already have a plan with ....to do....(It's not because I don't want to help on this. I have a plan first, so I can't help you)

They will be so disappointed if I can't be with them. The plan cannot be cancelled (further reason)

I can and will finish the work next week. (suggesting overwork is not necessary and give her a promise)

"Sorry, but I have a previous comittment." That lets the other person know that I'm busy, but it doesn't tell them if I am busy with a volunteer project, a date, other work, or something else.

It's quite funny to hear this question, as it's hard for me to understand how can anyone ask you to overwork in your free time. But maybe that's normal in China. I think, your weekend is your time and you're the only one to decide what to do with it. If you have your plans, you should say something like "I'd like to help you on Monday/Tuesday/..., but I have an appointment for Saturday and usually I'm staying with my family/friends at the weekends. If I won't go, it will hurt their feelings. I could help you if you asked earlier, on Friday, for example." - something like this (I don't know your situation and your relationship with that leader, I also don't know how much is that work important for you and whether you can lose it by rejecting it). 
Can you explain me, who is that "leader" and what is that work exactly? Is it part time job or some school work?