A normal of temperature, precipitation or any other weather element is defined as the arithmetic average of the observed values of that element.
There is an international agreement that averages or normals should be from a 30-year period from 1961 to 1990. To calculate the normal daytime high temperature for say Winnipeg on September 15, we would add together all the daily high temperatures on this day over the 30-year period from 1961 to 1990, divide the total by 30, to get the average or normal.
Normals are supposed to be representative, not necessarily what should occur. In Canada, the atmosphere is rarely normal.