The onus is on the state to prove an allegation beyond a reasonable doubt at a criminal trial in Canada. If the necessary witnesses for the prosecution are absent on the trial date, the Crown may not be in a position to prove its case.
While a judge has a statutory discretion to adjourn a trial to another date, this authority is not exercised lightly. When considering whether to grant the Crown’s request for an adjournment, a judge will contemplate the following comments from the Supreme Court of Canada in R. v. Darville,  S.C.J. No. 82 at para. 13:
There was no disagreement before us as to what conditions must ordinarily be established….in order to entitle a party to an adjournment on the ground of the absence of witnesses, these being as follows:
(a) that the absent witnesses are material witnesses in the case;
(b) that the party applying has been guilty of no latches or neglect in omitting to endeavor to procure the attendance of these witnesses; (and)
(c) that there is a reasonable expectation that the witnesses can be procured at the future time to which it is sought to put off the trial.