You can and should prepare for the first season in 5 minutes

In Daiblo 4, all players must log in from July 18th, before Season 1 begins. Because the new update requires that you become active.

hamburg – Diablo 4 It finally starts in the first season. But two days before it starts, players must invest five minutes of their time and do something important. Blizzard causes confusion with the “mandatory” character login. If you want to move map progression to the new chapter using, say, the fun locations of Lilith’s Altars, you’ll have to manually activate it.

ingame.de explains why every character in Diablo 4 has to log in:

You need to be especially careful with your hardcore characters before the start of Season 1 in Diablo 4. If they die, progression will also be erased and cannot be taken over in Season 1. Of course, you don’t have to delete a character before the July 18th update (and sign in after downloading). Because even then, Diablo 4 completely forgets what the character saw and can’t carry anything over to Season 1 on July 20th.