How to customize the month cell appearance using the DataTemplate in WinUI Scheduler (Calendar)
Customize the default appearance of the month cell by using the MonthCellTemplate property of the MonthViewSettings in the WinUI Scheduler.
XAML
Set the scheduler ViewType property as Month view. Customize the background color of the month cell view by customizing the loaded element in the MonthCellTemplate.
<scheduler:SfScheduler x:Name="Schedule"
ViewType="Month">
<scheduler:SfScheduler.MonthViewSettings>
<scheduler:MonthViewSettings>
<scheduler:MonthViewSettings.MonthCellTemplate>
<DataTemplate>
<Border Background="BlueViolet">
<TextBlock HorizontalAlignment="Center" Foreground="White" Text="{Binding DateTime.Day}"/>
</Border>
</DataTemplate>
</scheduler:MonthViewSettings.MonthCellTemplate>
</scheduler:MonthViewSettings>
</scheduler:SfScheduler.MonthViewSettings>
</scheduler:SfScheduler>

Take a moment to pursue the documentation. You can also find the options like customizing the month cell appearance.