Ex Post Facto — After the fact. In criminal law, a retroactive law, that changes the legal consequences or status of actions that have been committed, is “ex post facto.”
Section 11(g) of the Charter of Rights and Freedoms makes ex post facto criminal laws unconstitutional. If the punishment for a crime is changed between the time the crime was committed and sentencing, the lesser punishment applies.
Crothall Employees Ongwanada Hospital Penrose Div. 752 King St. W. v. Canadian Union of Public Employees, 1984 CanLII 949 (ON LRB)
“The case at hand bears some similarity to an ex post facto reward. The employees who circulated the petition so successfully got two days off work and received an all expense paid trip to Toronto to press their case for the termination of the respondent’s bargaining rights.”
Rosen v. R., 1979 CanLII 59 (SCC),  1 SCR 961
“The case should nonetheless be disposed of by ordering a new trial on another ground, namely, the inducement to give the ex post facto consent by a promise of leniency which excluded imprisonment and which was redeemed.”