Concert day: lightsticks, the merch queue, photocard trades

The official lightstick pairs over Bluetooth in the venue and changes colour with the songs. You can enjoy the show without one, but the moment the whole floor turns the same colour lands differently when you are holding one.
Merchandise queues form hours before the doors open, and the popular items can be gone by late morning.
If there is an online pre-order with pickup at the venue, take it — it is far faster. The details usually appear in the concert notice a few days ahead.
Outside the venue, fans trade photocards with each other. Stand with a sheet of paper naming the member you want and the conversation starts itself.
Some fan accounts hand out slogans and banners for free. If you take one, you hold it up with everyone else at the cue during the show.
Filming rules differ by concert. Short photos are usually fine and video is usually limited — read the notice at the entrance on your way in.