I would allow at least 2-3 months in advance for booking flights on the prefered date you have indicated. So purchase by the end of September or end of October. December 27 is post X-mas so many travellers don't pack the flights to go home until New Year or the day after New Year. Usually December 25-31 are not as heavy as December 12-24 or Jan 1-3. Both AC and KL have direct flights between YYZ-AMS.