Why do airline schedules start in March and October?
The main reason for new schedules in March and October is the Daylight Savings Time changeover. 

It is a MAJOR headache for airlines all over the world. Schedules have to undergo extensive restructuring during the switch over from summer daylight savings time to winter standard time. Maintaining code-share connection times is the biggest priority. 

What makes this all very difficult is the fact that many countries and localities do not observe daylight savings time, or have different dates for the switch-over. This is especially true for flights between northern and southern hemispheres.