The expiration date, or the beyond use date, is the date after which the medication is no longer as effective. These standards are determined by the United States Pharmacopeia to ensure quality of safety and potency. If the manufacturer only puts a month and year for the expiration date, it is the standard to refer to the last day of the month.
Therefore, when the expiration date on your birth control say "June 2018" it expires on June 30, 2018. The pack of birth control is still effective if it has been stored properly in the original packaging.
The USP also sets the standard for storage conditions that are required for medications. Make sure you store the birth control in the pack and at room temperature. Here is another article that describes how to store birth control, birth control left in the sun.
It is important to check the expiration date before taking any medication. A common mistake that could be made when dispensing birth control occurs when patient receives a 90 day supply or more of medication at once. The dispensing pharmacy might not realize that the medication may be out of date by the time you are ready to start the monthly pack.
We also wrote about taking expired birth control pills, which can be found here.