The normal rule is that, besides the arcane keep, only one keep is open at a time - but during the first week, they are exceptionally all open so very new players can progress faster. 

The reasoning is pretty simple - to push you to play every single day if you want to progress, instead of just whenever you feel like it. And it adds an element of planning to the game - giving an edge to people who think ahead and already get potions a few days before they'll actually want to use them, because that's when the keep is open. 

Thanks for the info. 

.