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The purpose of a taster day is to experience a regular college day to help you make the choice about the college you want to go to.
On a taster day you'll have the subjects you applied for. You bring your own equipment. On my college taster day they have been very nice and paid for transport and food.
You do not need to know anything about your subjects. That's the purpose of college to go and study and learn about your subjects. You might feel the need to look over the specification and buy some revision guides to look over but nothing serious.