Many summer special trains have already been announced. You can go to the "New Trains" subtab,
/trains/new to see all the new trains that have been included in this site - many of which are summer special trains, service commencing some time in April. Also, you can type "summer" in the Train drop down box above to see all the summer special trains. Against most of these special trains, you can also see the dates when the service of that train will begin and end.