The CASPer can be sat at a range of times throughout the year. As an example, if one were applying for Medicine in the US, one would likely be able to sit the exam from around the 7th July to May 4th the next year, with a total of around 30 exam times to choose from in this period. There is a far greater concentration of exam times from July to October than there are throughout the rest of the year, which fits with the application cycle and thus the majority of schools’ application deadlines.
The dates available will depend on three factors – the course that you are applying to, where you are applying to, and the language in which you will sit the exam. There is a helpful tool on CASPer’s website that allows you to enter the colleges that you are applying to, and the program that you have chosen, in order to provide you with the last possible date on which you might sit the exam. It takes into account each school’s distribution deadline.