It is important to always know when your I-20 is expiring. You can not let your program end date on your I-20 pass if you still need more time to complete your degree. Otherwise, you will be in violation of your F-1 status. 

The expiration date of your  I-20 can be found in the third box of your  I-20 in the section called "PROGRAM OF STUDY" on the first page.