As explained in the previous Clash of Clans' Clan Wars overview, each clan war is split up into two phases: Preparation Day and Battle Day. Each phase lasts 24 hours and allows you to perform different things.
The first phase, Preparation Day is when your clan prepares their defenses, gathers information about the enemy, and starts making plans of attack. During this phase there is no attacking. Instead, there are three main activities during preparation day: rearranging your war base, donating reinforcement troops to your clan mates, and scouting enemy war bases.
As a reminder, your war base is automatically generated to match your village layout. So if you want to rearrange your war base, just rearrange your regular village; but, be aware that you war base can not be rearranged after prepartion day has ended.
Also, the clan castle in your war base is different than the one in your village, so it must be filled seaprately. Troops donated to a war base will be used to defend it against all attacks during battle day. Unlike you rregular village though, there is no need to request troops for your war base; they are automatically open. Specific troops can be requested if you wish however.
Lastly, during Preparation Day you are free to scout all allied and enemy war bases to view their layout. "Scouting is a crucial tool for clan war strategy, as it allows you to give advice to allies and plan in advance for attacks against the enemy," Supercell says.
After Preparation Day, comes the second phase, Battle Day which you can read more about here.
