From Statistics Explained
The population figure of a given area is the total number of people in that area at a given time.
For the population data collected by Eurostat from the European Union (EU) Member States data that time is 1st January. The recommended definition is the 'usual resident population', representing the number of inhabitants of a given area on 1st January of the year in question (or, in some cases, on 31st December of the previous year). The population can be based on data from the most recent census adjusted by the components of population change produced since the last census, or based on population registers.
The average population during a calendar year is calculated as the arithmetic mean of the population on 1st January of two consecutive years. The average population is further used in the calculation of demographic indicators, like the crude rates per 1 000 inhabitants, and for some 'per capita' indicators.