In August of 2015, Citizenship and Immigration Canada (now Immigration, Refugees, and Citizenship Canada - IRCC) implemented new rules requiring most passport-visa-exempt nationals to seek an Electronic Travel Authorization (ETA) prior to travelling to Canada by air. These rules were implemented by the former Conservative Government, just prior to the 2015 election win of the current Liberal Government.
Following industry input, the new government has announced that they are postponing the mandatory ETA requirement for an additional six months. The rationale is that there is concern that there has been insufficient publicity of the requirement to date and that mass refusals could have a detrimental impact on business in Canada. IRCC has announced that Canada Border Services Agency officials at airports are advised to allow the admission of those without ETAs after March 15th, the previously announced date for mandatory ETAs. Presumably, airports and airlines around the world have been given the same instruction, as it would have been the responsibility of those organizations to bar entry onto planes without valid ETAs.
At this time, ETAs are scheduled to become mandatory in the Fall of 2016, presumably mid-September. Although the news of this is currently published on some places on the government web site, there are places where the March 15th date remains mentioned (notably, the ETA application page at present).