Hi there!
Not a UoL student, so my best advice would be to contact them directly, however I would assume that throughout their applicant day for major/minors, there will be sessions for each subject that is offered under this - meaning that there will likely be a session for law and also a session for criminology. I believe they run the day like this because there are lots of different major/minors at the uni, so some students would major law and minor a different course to criminology, so it is easier to have separate sessions for each subject. PLEASE confirm this with them yourself though before signing up!
Also, if you haven't visited Liverpool before for an open day, I would make time to take a trip up any way to decide whether you like the city or not before making a final choice - the applicant day would be a great way to do this because you can visit the Uni and spend the rest of the day in Liverpool too! If budget allows, I recommend staying for the night, considering how far away you live, as it will give you more time to explore and rule out rushing around to get there and back in one day
Hope this helps!
Jorja (LJMU Rep)