It took me a good while but I found this on Google.
The Japanese Language Proficiency Test (JLPT) is offered once a year in December. It is the best-known test of Japanese and has four levels, with Level 4 being the easiest and Level 1 being the hardest. You can “challenge” any level you choose, but if you try a level that is beyond your ability, but you must pass the test to be accredited for that level. Although anything less than Level 2 is unlikely to be of much help on your resume, studying for Levels 3 and 4 can be a good method of focusing your studies. If you hit the ground running when you arrive in Japan, it’s entirely realistic to pass Level 4 by December of your first year. Then you can challenge successive levels each year you are on the JET Programme. If you plan to challenge the test this year, make sure to get you application early as the deadline is September 3!