Primark assessment days are quite new and therefore the format may not be completely established yet. It is likely that there will be a maths test, group exercise and interview.
The maths test is likely to be checking that you have the necessary skills to be able to work the till, check change and things like that so it would be arithmetic. It is very likely that no calculators will be allowed and so making sure that you are up to sped with this sort of question is wise. Try to do some practice if you can, even if you are confident in your abilities. Then you will have more confidence when you are doing the test.
The group exercise is probably to find out what your leadership skills are like as well as how well you work with a group. If you are with other strong personalities, then it will not be possible for you all to be leaders. However, there are things that you can do to demonstrate those qualities, such as taking notes, leading some of the conversation, making decisions, summarising things etc. Do your best to make sure that everyone in the group has the opportunity to speak, including yourself.
You will have an interview probably with one or two HR people. You will need to therefore make sure that you save some energy for this as it is likely to come towards the end of the assessment. You will need to prepare so that you can answer questions about the company, why you want to work there, what makes you suitable for the role etc. Preparing is important because not only does it make you sound more knowledgeable but it also gives you more confidence in your ability to do well in the interview.