For many years the IATA international summer to winter timetable change has been the last weekend in October, and the winter to summer change the last weekend in March.

In 2001 the EU brought in a harmonised date of change of summer and winter times across member countries.This happens to be the same weekends that IATA uses. As new countries join the EU they have to agree to adopt the same dates.

So as other posters have pointed out, except where routes are affected by curfews or shortened vital connections in non-EU countries, the clock change doesn't affect the flight departure times, it is the IATA seasonal timetable change that is the driver.