If I understand correctly, you want an additional field linked to your existing Year and Month fields, to group the data in pre post July classes. Create this field in the load script, then use a list box to select your values:
if(Month < 7, Year & ' pre july', Year & ' post july') as Season
This will give you a very good insight on what exactly Qliksense does and its capability. There are umpteen number of documents in the Qlik Sense Resource Library which will help you understand most of the technical aspects of it.