The University Clinical Aptitude Test (UCAT), formerly known as the UKCAT, is a 2 hour multiple-choice admissions test used for medicine and dentistry applications by the vast majority of universities in the UK. The content is not medicine related – it is designed to test whether medical school applicants have the right skills to become a doctor.
The UCAT is an online test, and is usually done at designated test centres around the world (for UCAT 2020, the option to do the test at home has been made available). The testing session runs from July to October; due to Covid-19, the testing session will be from August 3 to October 1 for the 2020/21 application cycle. It is recommended for students to take the UCAT in the early half of the testing window as students will be preoccupied with other components of their applications such as the personal statement and other admissions tests like the BMAT in the later months. An additional advantage of taking the test early is that since you will receive your score as soon as you finish your test, you can plan your college options more effectively.
Do note, you are only allowed to take the test once in an admissions cycle, with the results being valid for only the admissions cycle in which the test was taken.