Why do my ears ring the day after a loud concert?

Ear damage. The loud noise causes damage to your ears which you hope is temporary. Unfortunately, in some people the damage is permanent and regrettable. Please protect your ears from excessive noise. If you are in a noisy environment and the noise is so loud you have to shout to be heard by someone next to you - you have about 5 minutes before you are at risk for permanent ear damage.
Noise trauma. It is the ear's response and warning to loud noise exposure. The ringing or tinnitus should go away after 24-48 hours. It persists beyond that period of time you should be evaluated with hearing test to check for any changes in your hearing.