Scheduled airlines bring there Summer schedule into effect from Sunday morning. The first Aer Lingus flight from DUB-HAM,SVQ,MRS and NAP will all commence on Sunday or throught the week as well as ORK-MUC,FAO, and 2 others. Saturday will also see the last EI flights to MXP as they will switch to Bergamo.

HLX will also up DUB-HAM to daily. And OS are back I think.

However charters dont start until May and either does US to PHL.
Airports dont decide when to start summer season.
The airlines do and the change 99% of the time the change takes place on the last sunday in March and October.
I shall be reciving my Summer timetable in the post any day now.