May 18th this year for what is usually referred to as the 'summer' timetable. I think it is usually the nearest Sunday to the middle of the month.
But officially this is only what they call the 'subsidiary change', the December change is the main annual change, by agreement with the EU. As far as a normal passenger is concerned there is little or no difference.
Thanks for the info. As a northern rail passenger I see there is a lot of changes from this date this year. I remember last year like you said I seen little or no difference.