Well - it works - but not quite as planned.
I used 'Model No', I gave Tickets a model no. in the form
xxxx-yy
yy is 01/02/03/04 - Adult/Child/Senior/Family
xxxx is Gen/East/Whit/etc - General/Easter/Whit/ etc to indicate which event
I added a radio button type attribute (I have only done easter - I know its a bit late but this is for future use)
to select Sunday or Monday
I tried not setting a default and making the options required - but that doesn't seem to work - you can proceed without making either selection, so I have set a default

The confirmation email and the invoice show the model no. Identifying the event, the description and model no. give the ticket type and the radio button selection is included which shows which day