Seats at the concert are limited and are on a first come, first seated basis. The best news though, is that the concert will be broadcast live.
The concert starts at 12:30pm and can be heard at the following link. Near the top of the page, next to the P4 Stockholm logo, is a purple box with a purple line titled JUST NU. Just click on the box and an online radio should appear at the bottom of your browser.