Usually the summer before or during their Junior years before they apply that following June.
Non-traditional students (like me still in college) looking to take a gap year the summer before their senior year or during their senior year to apply that following June
I would highly recommend taking it during the Summer (Like I did 7/21) because you have dedicated study hours, and once you pick a date, stick to it. I know like five people who wanted to take it over the summer but are still preparing for it because they keep pushing the test date back.