How much notice is your employer required to give you in order to mandate an unscheduled work shift?

There's no legislative standard. It would be a function of the terms of the contract, written or not. If you're a member of the bargaining unit, then check the terms of the collective agreement. If there's nothing in there, or there's no written agreement that applies to your engagement, then context would mean a lot.

In any event, there being no legislative standard, there's no argument you can make that the request itself is illegal, so the only question that remains is this: What can the employer do if you refuse?

I doubt that refusing a last-minute request for unusual hours would ever constitute just cause for dismissal, especially if there's a good reason for the refusal, unless it was previously known and understood that unusual hours might be required.

However, as a temp, you are probably not protected by the just cause analysis - if the employer wants you gone, they just tell the agency that, no?

you could do your own leg work since nobody in this forum knows if you are in a union or what not. Dont be lazy.