hey. a student can apperar for cat only when he is under some under graduate program or has completed it. CAT is basically for getting admissions into the top colleges of MBA.the exam is basically the test of your overall skills.It contains quantitative ability(QA),verbal ability(VA) and Reading comprehension(RC),Data interpretation(DI) and logical reasoning(LR). the exam i held every year in the month of november.for the exam pattern visit - https://bschool.careers360.com/articles/cat-exam-pattern

Hi

CAT or the Common Admission Test is given in order to secure admission into MBA colleges.The test scores a candidate on the bases of Quantitative Ability (QA), Verbal Ability (VA) and Reading Comprehension (RC), Data Interpretation (DI) and Logical Reasoning (LR). The Indian Institues of management (IIMs) started this exam and use the test for selecting students for their business administration programs. The test is conducted every year by one of the IIMs based on a policy of rotation.

So it is given either in your final year or graduation or after completing your graduation.

The paper is considered to be one of the toughest exams to crack in India. 

The pattern is explained in detail in the given link - https://bschool.careers360.com/articles/cat-exam-pattern.

All the best!