You can set CalendarDayButton's style for Calendar in the style , do not include the MouseOver state in the CommonStates under the VisualStateGroup like the below code show:
<VisualStateGroup Name="CommonStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0:0:0.1" />
</VisualStateGroup.Transitions>
<VisualState Name="Normal" />
<VisualState Name="Pressed">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="HighlightBackground"
Storyboard.TargetProperty="Opacity"
To="0.5"
Duration="0" />
</Storyboard>
</VisualState>
<VisualState Name="Disabled">
<Storyboard>
<DoubleAnimation Storyboard.TargetName="HighlightBackground"
Storyboard.TargetProperty="Opacity"
To="0"
Duration="0" />
<DoubleAnimation Storyboard.TargetName="NormalText"
Storyboard.TargetProperty="Opacity"
To=".35"
Duration="0" />
</Storyboard>
</VisualState>
</VisualStateGroup>