This is the day when your 2nd of 4 annual payments of self-employment tax are due.
The payment today is for taxes due on your net operations income generated during April and May.
Here are two links for more information, from BHB Advisors here in the Twin Cities:
Determining if this payment requirement applies to you
Details on how to make the payment.
Feel free to comment with any further questions or mentions of anything else that would be useful.